plz help to me, actually I cant find out how to got ans.

the pdf is given as

f(x, y) = 2, if (x,y) є R

else 0

there is marginal pdf's of X and Y

= 2y

then

E(X) = integral 0 to 1, x2(1-x)dx = 1/3

now can you describe this stepwise

E(X) = mathemetical expectation

becoz I cant analyse it