Let Y and Z be two independent continuous random variables with common probability density function f(x):

$\displaystyle 2e^{-2x}$ if $\displaystyle x \geq 0$

$\displaystyle 0$ if $\displaystyle x<0$.

How do you calculate the joint probability density function? I know how to do it the other way round, but I cannot see how to do this?

Thanks