how did my teacher get 11 for #17

Question

Answer:

Complementary angles ⇒ the angles add up to 90°

∠A and ∠B are complementary ⇒ ∠A + ∠B = 90°

∠A + ∠B = 90°

Plug in the values for ∠A and ∠B:
(3x - 8) + (5x + 10) = 90

Open the brackets:
3x - 8 + 5x + 10 = 90

Combine like terms:
8x + 2 = 90

Subtract 2 from both sides:
8x = 88

Divide both sides by 8:
x = 11

Answer: x = 11