Help with a density function

Question:

Suppose that random variables X and Y have a joint density function given by

What I have done so far:

Frankly I have been staring at this problem for a while. I'm not sure where to start. I cannot apply the following definition for joint probability functions

as the probability function is not independent. Anyone care to point me in the right direction? Once I could get it figured out, I can solve for b.

PS. Please don't post any solutions as it defeats the whole purpose of asking for help, I won't learn anything that way.