Hello,
When you have a polygon inscribed in a circle, the opposite angles are supplementary.
So we can start by saying that A + C = 180. Or x + 3x = 180. This gives us a value of 45 for x.
If we input that in for angle B, we get: 2(25) - 10 = 80.
If angle B = 80, then angle D must be 100, because the angles are supplementary.
Good luck,
MrEQ