For each of the following matrices, use Gaussian elimination on the augmented matrix $(A \mid \mathbf{b})$ to determine a basis for its cokernel:
(a) $\left(\begin{array}{ll}9 & -6 \\ 6 & -4\end{array}\right)$,
(b) $\left(\begin{array}{rr}1 & 3 \\ 2 & 6 \\ -3 & -9\end{array}\right)$,
(c) $\left(\begin{array}{rrr}1 & 1 & 3 \\ -1 & 1 & -2 \\ -1 & 3 & 6\end{array}\right)$,
(d) $\left(\begin{array}{rrr}1 & -2 & -2 \\ 0 & -1 & 3 \\ 2 & -5 & -1 \\ -2 & 2 & 10\end{array}\right)$.