the sum of the interior angles is 180° (triangles)
it's isosceles, 2 equal sides and 2 equal angles.
you take away the given angle from 360° and you will find the sum of the 2 equal sides, you divide by 2 and you have the angle b (equal to angle C).
(180 - 72) : 2 =
108 : 2 = 54° (your answer)
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72 + 54 + 54 = 180°
The answer is good.