That's a lot like asking: "What is Ax= 4?" without telling us what A is. The only possible answer is "x= 4/A" or "X= A^{-1}4".
The only possible answer to your question is
[tex]\left[\begin{array}{c} b \\ y \end{array}\right]= A^{-1}\left[\begin{array}{c} 3 \\ 4\end{array}\right][/tex]
until we know what "A" is.
Of course, if the problem really was what you said, that may be the correct answer!