Row Reduction & Rank · Easy problem

To solve $A\mathbf x = \mathbf b$ with $A = LU$, one first solves $L\mathbf c = \mathbf b$ and then $U\mathbf x = \mathbf c$. Suppose the first step gave $\mathbf c = \begin{pmatrix}5\\6\end{pmatrix}$ and $U = \begin{pmatrix}2 & 1\\0 & 3\end{pmatrix}$. Find $x_1$.