Find a constant C C such that p ( x , y ) = { C x y if 0 x and 0

Find a constant $C$ such that

$$p(x,y)=\{\begin{array}{cc}Cxy& \text{if}0\le x\text{and}0\le y\le 1-x\\ 0& \text{otherwise}\end{array}$$

is a joint probability density function. Then calculate:

(a) $P(X\le \frac{1}{2};Y\le \frac{1}{4})$

(b) $P(X\ge Y)$