I have no idea how to approach the following problem:

Random Variable $\displaystyle X$ has density function

$\displaystyle f(x) = \left\{ \begin{array}{ll}

ke^{-hx}, & x \geq 0 \\

0 & x<0 \end{array} \right.$

Find the value of $\displaystyle k$ and determine $\displaystyle E(x)$.