Answer:
(c) 20
Explanation:
You want to find x, given that the external angle adjacent to a base angle of an isosceles triangle is (5x +12)°, and the other base angle is marked (3x +8)°.
Base angles
The base angles of an isosceles triangle are congruent, so the other base angle is congruent to the marked base angle.
The external angle and the adjacent base angle are a linear pair, hence supplementary.
(5x +12)° +(3x +8)° = 180°
8x +20 = 180 . . . . . . . divide by °, collect terms
8x = 160 . . . . . . subtract 20
x = 20 . . . . . divide by 8