A and B are Vertical Angles and are thus congruent.
A = B
3y - 24 = 51
3y - 24 + 24 = 51 + 24
3y = 75
3y/3 = 75/3
y = 25
We can solve for A, but we already know A = B and B = 51, so A = 51.
C and D are also Vertical Angles. Since these angles go all the way around, they add up to 360.
51 + 51 + C + D = 360
C = D
C = x
D = x
D + C = 2x
102 + 2x = 360
102 - 102 + 2x = 360 - 102
2x = 258
2x/2 = 258/2
x = 129
C = 129
D = 129