Question

Angle a​ and​ ​angle b​ ​are complementary angles. The measure of ∠a is 42°.

Answer 1

Remember that complementary angles are the angles whose sum is 90 degrees, so for example if angle $\alpha$ and angle $\beta$ are complementary; then $\alpha + \beta =180°$.
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:
$42+x=90$
We can conclude that the equation that represents this situation with x representing the measure of ∠b is 42+x=90.