Then the second equation is ∠2 = (3 * 15 + 30)° which simplifies to 75°
And since complementary angles need to equal 90° to be true we have to add both angles. So, ∠1 (= 15°) + ∠2 (= 75°) = 90°. Now that we know they equal 90°, 15 would be the answer to your problem.
