So first of all, find d. It is the only letter you can be sure of because all of them are the same. So 3d = 9. Divide by 3 to get d = 3. Now 2C + 3 = 21, because there are 2 C's in the bottom row. Subtract 3 from both sides to get 2C = 18. Divide by 2 to get C = 9. 2C + A = 21, or 18 + A = 21. Subtract 18 from both sides to get A = 3. A + B + C = 14, or 3 + B + 9 = 14. Combine like terms to get 12 + B = 14. Subtract 12 from both sides to get B = 2. So A = 3, B = 2, C = 9, and D = 3.
