
William W. answered 12/05/20
Experienced Tutor and Retired Engineer
I'm assuming you mean m∠C = 3x + 9 and m∠D = 2x + 24.
Since the triangle is isosceles, the two base angles are congruent so we can say m∠C = m∠D which means 3x + 9 = 2x + 24 [subtract 2x from each side to get:
x + 9 = 24 [subtract 9 from both sides to get:
x = 15.
So m∠C = 3(15) + 9 = 54° and m∠D also = 54°
Since there are 180° in a triangle m∠E = 180 - 54 - 54 = 72°