(a)
$3a\int_{0}^{1}\left( \frac{ax - 1}{a - 1} \right)^{2}dx = \frac{3a}{(a - 1)^{2}}\left\lbrack \frac{(ax - 1)^{3}}{3} \times \frac{1}{a} \right\rbrack_{0}^{1}$
$= \frac{1}{(a - 1)^{2}}\left\lbrack (a - 1)^{3} + 1 \right\rbrack$
$= (a - 1) + (a - 1)^{- 2}$