Final answer:
The measure of a supplement of ∠B, which is x°, is found by subtracting x° from 180°.
Step-by-step explanation:
If the measure of ∠B is x°, the measure of a supplement of ∠B would be the difference between 180° and x°. To find this, you simply subtract the measure of ∠B from 180°. Therefore, if m∠B = x°, the supplement of ∠B would be 180° - x°.