TITLE: Solve for unknown matrix
QUESTION [5 upvotes]: Let $A = \begin{bmatrix}
2 & 3 \\
4 & 5        
\end{bmatrix}$
and let $B = 
\begin{bmatrix}
3 & 4 \\
5 & 6        
\end{bmatrix}$
Solve $A X = B$ for a matrix $X$
My guess is that i: 
let $X = 
\begin{bmatrix}
x \\
y        
\end{bmatrix}$
Then solve it using a linear equation but i'm not too sure. Any help would be appreciated.

REPLY [5 votes]: Let 
$$X=\left[\begin{matrix} a &b\\ c &d\end{matrix}\right].$$
Expanding the matrix product, you need to solve
$$\begin{cases}2a+3c=3\\4a+5c=5\\2b+3d=4\\4b+5d=6\end{cases}$$
Hint:
$$\begin{cases}2a+3c=3\\4a+5c=5\end{cases}$$ $$\begin{cases}2b+3d=4\\4b+5d=6\end{cases}$$

The real, efficient method is by solving the system (by Gaussian elimination) for all right-hand sides simultaneously.
$$\begin{cases}\begin{align}2x+3y=3|4\\4x+5y=5|6\end{align}\end{cases}$$
$$\begin{cases}2x+3y=\ \ \ 3|\ \ \ \ \ 4\\0x-1y=-1|-2\end{cases}$$
$$\begin{cases}2x+0y=\ \ \ 0|-2\\0x-1y=-1|-2\end{cases}$$
$$\begin{cases}x=0|-1\\y=1|\ \ \ \ \ 2\end{cases}$$