I'm stuck at this question, if anyone could help please.

Show k = 24, -- I proved this by taking the double integral and setting = 1.

Find the marginal probility density function of X and Y,

Find the pdf of U = X + Y, X + Y <= u ( 0 < u < 1)

My marginal pdf for X was

and for Y, was

Is this correct, and if so how would I find U = X + Y, do I integrate

Also my epected value for E(X) and E(Y) was = 1/5