Explanation

(a)
Here, $\lim_{x \rightarrow - 3}x^{2} + 2x - 3 = 0$
$\therefore\lim_{x \rightarrow - 3}3x^{2} + ax + a - 7$ must be zero, in order to limit exist.
$\Rightarrow \ \ \ \ \ \ 3( - 3)^{2} + a( - 3) + a - 7 = 0$
$\Rightarrow \ \ \ \ \ \ \ 27 - 2a - 7 = 0$
$\Rightarrow \ \ \ \ \ \ \ \ 2a = 20$
$\Rightarrow \ \ \ \ \ \ \ \ a = 10$