(b)
Given, $f(x) = \left( a - x^{n} \right)^{1/n} = g(x)$
$\therefore\ \ \ \ \ \ \ \ fof(x) = f\left( f(x) \right)$
$\ \ \ \ \ \ \ = \left\lbrack a - \left\{ \left( a - x^{n} \right)^{\frac{1}{n}} \right\}^{n} \right\rbrack^{1/n} = \left\lbrack a - \left( a - x^{n} \right) \right\rbrack^{1/n} = x$