Answer:
m∠b = 40°
Explanation:
∠a and ∠b are complementary angles. Complementary angles form a right angle and have a sum of 90°.
Therefore we have the equation:
m∠a + m∠b = 90°
Substitute m∠a = 50°:
50° + m∠b = 90° subtract 50° from both sides
m∠b = 40°