A = 6
B = 9
C = 15
D = ?
From A to B, 6 + 3 = 9
From B to C, 9 + 6 = 15
From C to D, 9 + x = ?
The first time, you added 3, for the second time, you added three to the three you are adding, making it 6. So for x, x = 6 + 3, which = 9
9 + 9 = 18
D = 18 should be your answer
hope this helps