Escribí en un orden pero aparecieron en orden inverso. Por favor ayuda

Question

Este código consta de dos partes. Primero están las cifras y después los cálculos. (Indicado por los dos comentarios). Pero cuando los ejecuté aparecieron en orden inverso.

\subsection*{Figures}
\begin{center}       
%should appear first
    \begin{figure}[ht]
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/Ptime.jpg}
        \caption{Pressure vs Step}
        \vspace{4ex}
        \end{subfigure}
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/temptime.jpg}
        \caption{Temperature vs Step}
        \vspace{4ex}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/KEtime.jpg}
        \caption{Kinetic Energy vs Step}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{../figures/PEtime.jpg}
        \caption{Potential Energy vs Step}
        \end{subfigure}
    \caption{}
    \end{figure}
    \end{center}
    %should appear next
    \textit{Calculation of Self Diffusion Coefficient}
    \begin{equation*}
    \begin{split}
    \text{slope} & = 0.0021 \\
    \text{Self Diffusion Coeff} & =\frac{1}{6}\times \text{(Slope of MSD at long time)}\\
    & = \frac{1}{6} \times \text{0.0021}\\
    & = \SI{3.5e-4} {\text{LJ units}}
    \end{split}
    \end{equation*}
    \text{Considering the LJ liquid to be liquid Ar, we have $\sigma = 0.34 \textit{ nm}$, $\frac{\epsilon}{k_B}=120 \textit{ K}$. }\\
    \text{$\therefore \frac{{(L^*)}^2}{t^*}=\frac{{L^2}/{\sigma^2}}{t\sqrt{\frac{\epsilon}{m\sigma^2}}}\implies \frac{L^2}{t}= (\sigma^3\sqrt{m/\epsilon}) \frac{(L^*)^2}{t^*} \implies \frac{L^2}{\epsilon}=\num{2.49e-31}\frac{(L^*)^2}{t^*}$}
    \linebreak
    \begin{equation*}
    \begin{split}
    \text{Diffusion Coefficient in SI Units} &= \num{3.5e-4}\times \SI{2.489e-31}{\meter^2 /\second}\\
    &=\SI{8.7e-34}{\meter^2 /\second}
    \end{split}
    \end{equation*}

Answer 1

Funciona correctamente una vez que el texto sobre la segunda ecuación se acorta a una sola palabra de texto considering(probablemente algún error en la ecuación), aparte de eso, se alinea correctamente

\documentclass[]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}   
\usepackage{mathtools}
\usepackage{amssymb,booktabs}
\usepackage{caption,subcaption}

\begin{document}
\subsection*{Figures}
\begin{center}       
%should appear first
    \begin{figure}[ht]
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Pressure vs Step}
        \vspace{4ex}
        \end{subfigure}
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Temperature vs Step}
        \vspace{4ex}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Kinetic Energy vs Step}
        \end{subfigure} 
        \begin{subfigure}[b]{0.5\linewidth}
        \centering
        \includegraphics[width=0.9\linewidth]{example-image-a}
        \caption{Potential Energy vs Step}
        \end{subfigure}
    \caption{}
    \end{figure}
    \end{center}
    %should appear next
    \textit{Calculation of Self Diffusion Coefficient}
    \begin{equation}
\begin{split}
TCU & = \frac{\text{purchasing cost} + \text{setup cost} + \text{holding cost}}{time} \\
    & = \frac{cy+K+h\frac{y}{2}t_0}{t_0}
\end{split}
\end{equation}
    \text{Considering the LJ liquid to be liquid Ar, we have $\sigma = 0.34 \textit{ nm}$, $\frac{\epsilon}{k_B}=120 \textit{ K}$. }\\
    \text{$\therefore \frac{{(L^*)}^2}{t^*}=\frac{{L^2}/{\sigma^2}}{t\sqrt{\frac{\epsilon}{m\sigma^2}}}\implies \frac{L^2}{t}= (\sigma^3\sqrt{m/\epsilon}) \frac{(L^*)^2}{t^*} \implies \frac{L^2}{\epsilon}=\num{2.49e-31}\frac{(L^*)^2}{t^*}$}
    \linebreak
    \begin{equation}
\begin{split}
TCU & = \frac{\text{purchasing cost} + \text{setup cost} + \text{holding cost}}{time} \\
    & = \frac{cy+K+h\frac{y}{2}t_0}{t_0}
\end{split}
\end{equation}
    \end{document}