Operating system: Ubuntu 14.04LTS; tool: TeXstudio; in addition, I also installed the complete texlive (sudo apt-get install texlive-full, it is relatively large, you can install the part as appropriate).
The sample code is as follows:
$$
\begin{array}{rr}
x_{1}-2x_{2}+x_{3}=& 0 \\
2x_{2}-8x_{3}=&-8 \\
-4x_{1}+5x_{2}+9x_{3}=&-9
\end{array} \eqno(3)
$$
增广矩阵
$$
\left[
\begin{array}{rrrr}
1 & -2 & 1 & 0 \\
0 & 2 & -8 & 8 \\
-4 & 5 & 9 & -9
\end{array}
\right] \eqno(4)
$$The effect is as follows: 
I won't explain much here, I believe you can understand, just pay attention to the {rrrr} next to {array}, this is used to control the alignment, r is right-aligned, l is left-aligned, and c is centered , you can set it according to the actual situation. In addition, pay attention to the manual setting of formulas and matrix numbers, which is done with the code \eqno(x). This method is invalid in the Markdown of "Youdao Cloud Notes".