I have the joint density function:

$\displaystyle

f(x,y)=\left\{

\begin{array}{lr}

k(2x+y)&0\le x\le 1;0\le y\le 2\\

0&otherwise

\end{array}

\right.

$

First, I had to find k. So I integrated for x with given limits and then for y with the given limits and ended up with k=1/4. Correct?

Now I have to find the joint cumulative distribution function... stuck. Do I just put in k=1/4 into the density function and then find the derivative?