Number 3 i don’t understand it

The measures of complementary angles add up to 90 degrees. Angles A and B therefore add up to 90 degrees.

To find the value of x, set the sum of (the measures of) A and B to 90 (degrees), and solve algebraically.

m∠A + m∠B = 90°
(3x + 5)° + (2x - 15)° = 90°

5x - 10 = 90
5x = 100

x = 20

Then, plug the value of x back into each expression to find the measures of angles A and B.

m∠A = (3x + 5)°
m∠A = (3(20) + 5)°
m∠A = (60 + 5)°

m∠A = 65°

m∠B = (2x - 15)°
m∠B = (2(20) - 15)°
m∠B = (40 - 15)°

m∠B = 25°

You can check to make sure that the angles are complementary by adding them together. Their measures should equal to 90°.

65° + 25° = 90°