Answer
(a) $\frac{1}{2}$
(b) It is not possible because we can not perform a division in which the denominator is equal to 0.
Work Step by Step
$\frac{x}{x-3}$
(a) $x=-3$
$\frac{-3}{-3-3}=\frac{-3}{-6}=\frac{1}{2}$
(b) $x=3$
$\frac{3}{3-3}=\frac{3}{0}$. It is not possible.
