Your equation is a good place to start. Let x represent the measure of the angle.
... (1/2)(90 -x) +21 = x
... 90 -x +42 = 2x . . . . . multiply by 2
... 132 = 3x . . . . . . . . . . collect terms, add x
... 44 = x . . . . . . . . . . . . divide by 3. This is the measure of the angle.
180 - 44 = 136
The supplement of the angle is 136°.