I forgot to tell you what I was trying to do !
well, I tried to find F(x) from f(x), so I can use a transformation:
F(Y)=P(Y<y)=P(E^X<y)=P(x<ln y)=Fx(ln y)
the thing is, that I can't find F from f, the integral is too hard.
so I left it and tried to use moment generating functions, there is a law saying that if Mx(t)=My(t) then X and Y have the same distribution. this idea failed too, maybe it's the correct way ( since it's a statistics course and not probability it make sense...) but the maths failed me once again, the integral is too hard and I couldn't solve it.
I am desperate for ideas...