I have problems with deriving the CDF of a weighted difference of iid uniformly distributed random variables.

Assume that $X\_1$ and $X\_2$ $\stackrel{iid}{\sim} U[0,1]$. Define Z = $a\cdot X\_1 - X\_2$, where $a > 0$. How can I derive $P(Z<z)$? I was struggling with applying the general definition of a convolution on my particular case...

Thanks in advance!