Complementary angles are angles whose sum is equal to 90°.
m∠1 = 5x + 4
m∠2 = 7x + 2
m∠1 + m∠2 = 90°.
5x + 4 + 7x + 2 = 90
12x + 6 = 90
12x = 90 - 6
12x = 84
12x / 12 = 84/12
x = 7 Choice B.
m∠1 = 5x + 4 = 5(7) + 4 = 35 + 4 = 39
m∠2 = 7x + 2 = 7(7) + 2 = 49 + 2 = 51
39 + 51 = 90
90 =90