Find the least squares solutions to the following linear systems.
(a) $\left(\begin{array}{rr}1 & -1 \\ 2 & 2 \\ 3 & -1\end{array}\right)\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{r}1 \\ 0 \\ -1\end{array}\right)$,
(b) $\left(\begin{array}{rr}3 & -1 \\ 0 & 2 \\ -2 & 1 \\ 1 & 5\end{array}\right)\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{r}2 \\ 1 \\ -1 \\ 1\end{array}\right)$,
$(c)\left(\begin{array}{rrr}1 & -1 & -1 \\ 1 & 3 & 2 \\ -2 & 1 & 0 \\ 1 & 0 & -1 \\ 0 & 7 & -1\end{array}\right)$
$\left(\begin{array}{l}x \\ y \\ z\end{array}\right)=\left(\begin{array}{r}-1 \\ 0 \\ 1 \\ -1 \\ 0\end{array}\right)$.