In each of Exercises $23-28,$ a function $g$ and an interval I are specified. The function $g$ is nonnegative on $I .$ Find a number $c$ such that $f(x)=\operatorname{cg}(x)$ is a probability density function on $I .$
$$ g(x)=e^{-2 x} \quad I=[0, \infty] $$