Answer:

Explanation:
For the two angles, ∠A & ∠B, to be supplementary angles,
m∠A + m∠B = 180°
Here:-
Thus,

(Combine like terms)
(Subtract 40 from both sides)

(Simplify)
( Divide both sides by 7)

Now that we know what x is, let's find the measure of ∠B:-

( Multiply:- 3 × 20 = 60)
(Add)

Therefore, the measure of ∠B = 71°.
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