(a)
Given, $f(x) = \lambda e^{- ax},for\ 0 \leq x < \infty\ and\ a > 0$
$\therefore\int_{0}^{\infty}{\lambda e^{- ax} = 1\ \ \lbrack\because}\ sum\ of\ total\ distribution\ is\ one\rbrack$
$\Rightarrow \ \lambda\left\lbrack \frac{e^{- ax}}{- a} \right\rbrack_{0}^{\infty} = 1$
$\Rightarrow \ \lambda\left\lbrack 0 + \frac{1}{a} \right\rbrack = 1 \Rightarrow \lambda = a$