In the following exercises, decide whether it would be more convenient to solve the system of equations by substitution or elimination.
(a) $\left\{\begin{array}{l}y=4 x+9 \\ 5 x-2 y=-21\end{array}\right.$
(b) $\left\{\begin{array}{l}9 x-4 y=24 \\ 3 x+5 y=-14\end{array}\right.$