Systems of Equations · Hard problem

Write the system as $A\mathbf{v} = \mathbf{b}$ with $A = \left[\begin{matrix}1 & 3\\-4 & 3\end{matrix}\right]$ and $\mathbf{b} = \left[\begin{matrix}-25\\10\end{matrix}\right]$. Find $\det A$, and then $x$ and $y$.