Angles DCE and ACB are vertical angles, hence equal. Both have the value 3x.
Angles ACH and HCB (x) add to give ange ACB, so they sum to 3x. That makes ACH have the value 2x. (2x +x = 3x)
The angles in triangle ACH add to 180°. This sum is ...
... 2x +2x +100° = 180°
... 4x = 80° . . . . . . . . subtract 100°, then divide by 4 to get ...
... x = 20°
