Sampling Distribution/estimation

[kangkan]

If X1 and X2 are the value of a random sample from an uniform population with a=0 and B= b..find k so that

0<b<k(x1+x2) is a (1- alpha)*100% confidence interval for b.

I tried to calculate the distribution function of (x1+x2) but it isnt using MGF but it is not a familair function :( any other methods?