A system consisting of one original unit plus a spare can function for a random amount of time $x.$ If the density of $x$ is given $($in units of months$)$ by

$f(x)=\{\begin{array}{cc}Cx{e}^{-\frac{x}{2}},& x>0\\ 0,& x0\end{array}$

what is the probability that the system functions for at least $5$ months?