We are given a system of equations,
![\begin{cases}-x+2y=0\\y=-2\\ \end{cases}](/tpl/images/1397/8489/41914.png)
This will translate into a 2x2 matrix of coefficients (because 2 equations and 2 unknowns),
![\begin{bmatrix}-1&2\\0&1\\ \end{bmatrix}](/tpl/images/1397/8489/cbb36.png)
The matrix will then be applied to the vector (lower dimensions on top),
![\begin{bmatrix}x\\y\\ \end{bmatrix}](/tpl/images/1397/8489/06a8a.png)
And the result vector will be whats on the other side of equals sign,
![\begin{bmatrix}0\\-2\\ \end{bmatrix}](/tpl/images/1397/8489/91ed6.png)
So to put everything together,
![\begin{bmatrix}-1&2\\0&1\\ \end{bmatrix}\begin{bmatrix}x\\y\\ \end{bmatrix}=\begin{bmatrix}0\\-2\\ \end{bmatrix}](/tpl/images/1397/8489/8917f.png)
Hope this helps :)