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