In every parallelogram, the closest angles are supplementary (adjacent angles), that is, they sum up to 180º. Since B and C are adjacent angles, we have:
∠C + ∠B = 180º
2x + 12º + 5x = 180º
7x + 12º = 180º
7x = 180º - 12º
7x = 168º
x = 168º/7 ≅ 24º
After we find x, we can use its value to find angle B:
∠B = 5 * x = 5 * 24º = 120º
Now, we use the fact that in every parallelogram, two opposite angles are congruent. So, since D and B are opposite angles, we have:
∠D = ∠B
∠D ≅ 120º