the constraint is
the two rvs are uniformly distributed over that triangle in the first quadrant
so on 0<x<1
THIS IS UNIFORMLY DISTRIBUTED on any line for a fixed x
so P(0<Y<3/4|X=0.5) is a silly question, the answer is 1.
3. Let the joint density of (X,Y) be given by f(x,y)=2, 0<X<1, 0<Y<1-X
a) Calculate marginal density f(x),
b) Calculate P(0<Y<3/4|X=0.5).