Given $A = \begin{pmatrix} 1 & 2 \\ 3 & 6 \end{pmatrix}$ and $b = \begin{pmatrix} 2 \\ 6 \end{pmatrix}$, which vector $x = \begin{pmatrix} x_1 \\ x_2 \end{pmatrix}$ is a solution to $Ax=b$?