Determinants & Inverses · Medium problem

A two-step chemical process relates input amounts of reagent $1$ ($x$) and reagent $2$ ($y$) to two target outputs via $\begin{pmatrix}2&1\\1&3\end{pmatrix}\begin{pmatrix}x\\y\end{pmatrix}=\begin{pmatrix}7\\11\end{pmatrix}$. Since the matrix's determinant is nonzero, a unique input combination exists. Find the required amount of reagent $1$.