Angle a and angle b are complementary angles. the measure of ∠a is 42°. which equation represents this situation with x representing the measure of ∠b ? 42−x=90 90+x=42 90+42=x 42+x=90

Remember that complementary angles are the angles whose sum is 90 degrees, so for example if angle  and angle  are complementary; then .
In our case we have an angle a whose measure is 42° and and angle b, repented by x, whose measure we don't know; similarly:

We can conclude that the equation that represents this situation with x representing the measure of ∠b is 42+x=90.