Remove indent in equation LaTex - latex

Question

I have a rather simple problem. I have an unwanted indent in my equation.
\documentclass[11 pt, a4paper, fleqn]{article}
\usepackage{float}
\usepackage{amssymb}
\usepackage{amsmath}
\setlength{\mathindent}{0cm}
\begin{document}
\noindent Through rigorous analytical calculations found in \textbf{Appendix A} a state space representation was found:
\begin{multline}
X= \begin{pmatrix}\dot{x_1} \\ \dot{x_2} \\ \dot{x_3} \end{pmatrix} =
\begin{pmatrix}0 & 1 & 0\\ 0 & 0 & 1 \\ -1000 & -300 & -30\end{pmatrix}
\begin{pmatrix}x_1 \\ x_2 \\ x_3 \end{pmatrix}+\begin{pmatrix} 0 \\ 0 \\ 160 \end{pmatrix}u \\
\noindent Y=\begin{pmatrix}1 & 0 & 0 \end{pmatrix} \begin{pmatrix}
x_1 \\ x_2 \\ x_3 \end{pmatrix}
\end{multline}
\end{document}
Which give me this result:
enter image description here
I would like to have the second line at the left margin.
I have tried using \[ \begin{aligned} \end{aligned} \] and at the before I begin my document I have the command \setlength{\mathindent}{0cm} but still the indent persists. It's driving me quite mad.
Please and thank you!
edit: made compileable

Answer 1

You can use:
\begin{align}
X &= \begin{pmatrix}\dot{x_1} \\ \dot{x_2} \\ \dot{x_3} \end{pmatrix} =
\begin{pmatrix}0 & 1 & 0\\ 0 & 0 & 1 \\ -1000 & -300 & -30\end{pmatrix}
\begin{pmatrix}x_1 \\ x_2 \\ x_3 \end{pmatrix}+\begin{pmatrix} 0 \\ 0 \\ 160 \end{pmatrix}u \\
Y &=\begin{pmatrix}1 & 0 & 0 \end{pmatrix} \begin{pmatrix}
x_1 \\ x_2 \\ x_3 \end{pmatrix}
\end{align}
Use align* if you don't want line numbers.
I hope that works!

Answer 2

The multiline environment is specifically meant for a single equation which is too long for one line. The first line will align to the left, the second to the right.
If you want to align multiple different equations, I would use something like the align environment as #HelloWorld shows in their answer.
However just for completeness, you can also do this with multiline:
\documentclass[11 pt, a4paper, fleqn]{article}
\usepackage{float}
\usepackage{amssymb}
\usepackage{amsmath}
\setlength{\mathindent}{0cm}
\begin{document}
\noindent Through rigorous analytical calculations found in \textbf{Appendix A} a state space representation was found:
\begin{multline}
X= \begin{pmatrix}\dot{x_1} \\ \dot{x_2} \\ \dot{x_3} \end{pmatrix} =
\begin{pmatrix}0 & 1 & 0\\ 0 & 0 & 1 \\ -1000 & -300 & -30\end{pmatrix}
\begin{pmatrix}x_1 \\ x_2 \\ x_3 \end{pmatrix}+\begin{pmatrix} 0 \\ 0 \\ 160 \end{pmatrix}u \\
Y=\begin{pmatrix}1 & 0 & 0 \end{pmatrix} \begin{pmatrix}
x_1 \\ x_2 \\ x_3 \end{pmatrix}\hfill
\end{multline}
\end{document}