78 is the answer
Explanation:
The two angles are complementary so m<A + m<B = 90
we are given m<A = 3x + 9 and m<B = x - 11
substituting we get 3x + 9 + x - 11 = 90
4x - 2 = 90
4x = 92
x = 23
m<A = 3x + 9 = 3(23) + 9 = 78
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