Find the least squares solution to the linear system $A \mathbf{x}=\mathbf{b}$ when
(a) $A=\left(\begin{array}{rr}2 & 3 \\ 4 & -2 \\ 1 & 5 \\ 2 & 0\end{array}\right), \quad \mathbf{b}=\left(\begin{array}{r}2 \\ -1 \\ 1 \\ 3\end{array}\right)$
(b) $A=\left(\begin{array}{rrr}2 & 1 & 4 \\ 1 & -2 & 1 \\ 1 & 0 & -3 \\ 5 & 2 & -2\end{array}\right), \quad \mathbf{b}=\left(\begin{array}{l}0 \\ 0 \\ 1 \\ 0\end{array}\right)$.