Hey gfbrd.
Hint: Try integrating out one of the variables (either x or y) and bring this constant outside the final integral (which will be with respect to the other variable). Once you do this, substitute the old integral in for the constant and you are done.
So as an example Integral Integral f(x)g(y)dxdy = c*Integral g(y)dy. What is c in terms of the integral of f(x)dx?