Im supposed to prove the following by using the Fundamental Theorem of Calculus twice...

$\displaystyle \int_{c}^{d}(\int_{a}^{b}\rho _{x}(x,y)dx)dy=...=\int_{a}^{b}(\int_{c}^{d}\rho _{x}(x,y)dy)dx$.

Along with the Fundamental Theorem of Calculus I also think that Fubini's Theorem might play a role in this proof...

I'm just not sure how to get the ball rolling. Any help would be appreciated.