I have a joint density function

$\displaystyle f(x,y)= 3x, 0 \leq Y \leq X \leq 1 $

And I have to find

$\displaystyle P(X \leq 1/2 , Y \leq 1/3)$

I integrated 0 to X dy and 0 to 1/3 dx

Obviously I'm doing something wrong since that is not the answer in the book. So a little help would be appreciated.