Finding expected value for random variable $X$ given a joint probability density function $f(x,y)$ I've been given $f(x,y) = 6y$ with boundaries $0 \leq y \leq x \leq 1$.
How do I find the expected value of $x$?

Calculate
$$\iint_D (x)(6y)\,dy\,dx,$$ 
where $D$ is the part of the plane such that $0\le y\le x\le 1$.
So $D$ is the interior and boundary of the triangle with vertices $(0,0)$, $(1,0)$. and $(1,1)$.
