Orthogonality & Least Squares · Medium problem

A least-squares estimate for a position $(x,y)$ is required to be consistent (zero residual) with both $x+y=3$ and $2x-y=0$. For which value of $k$ is $(x,y)$ ALSO exactly consistent with $x+2y=k$ (so all three constraints share a common least-squares solution with zero total residual)?