X2 and x1 being 2 independent exponential variables with mean 5 and 2.
I found f(x1) and f(x2) and defined y as x2 - x1 but I'm stuck there. How do I calculate f(y)? or is there another way of finding P(x1<x2).
Thank you!

I did not do it with the joint function. I integrated f(x1) from 0 to x2 and subtracted the result from 1.
I was wondering if I should have used y=x1-x2 instead and solve P(y>0)