(b)
We have, $\int_{0}^{a}{f\ (2a - x)dx = \mu}$
and $\int_{0}^{a}{f(x)dx = \lambda}$
Now, using properties of definite integral
$\int_{0}^{2a}{f(x)dx = \int_{0}^{a}{f(x)dx + \int_{0}^{a}{f(2a - x)dx}}}$
$\Rightarrow \ \ \int_{0}^{2a}{f(x)dx = \lambda + \mu}$