Bivariate random variables question

I got this question here and I have trouble just to start.

Consider the random variables which is uniformly distributed over the triangle .

(a)Write down , the disjoint probability density function of , and indicate on a graph the triangle where it is non-zero.

(b)Explain why X and Y are dependent (no calculations should be required)

I'm having trouble from the start to write down disjoint probability density function. I can surely draw the graph and see the triangle area, but how do I get density function? Can anyone please help me?