A machine consisting of 2 components will be damaged, if both components are damaged. Suppose T1 and T2 are the length of time the two components can work before being damaged. T1 and T2 are independent of each other and follow the following distribution: f(t) = e^-t ; t>0 = 0 ; other

Let X be the maintenance cost of the machine until it breaks down, equal to 2T1 + T2. Let g be the density function for X. Find g(x) for x>0

The answer is e^(-x/2) - e^(-x), please explain why??