Two angles are complementary if their sum is 90 degrees.
Then, we can write the following equation:
[tex]m\angle A+m\angle B=90\text{\degree}[/tex]
Now, we replace the measure of angle A in the above equation, and we solve for the measure of angle B:
[tex]31\text{\degree}+m\angle B=90\text{\degree}[/tex]
