Complementary angles are 2 angles who add up to 90 degrees. Every one of the angles adds up to 90 degrees, so the assignment is asking you to find the missing number that makes the angles add up to 90.
Example: The first angle on your pager shows 45 and and a question mark. Ask yourself, what number + 45 = 90? The answer is 45. So, take the number of each angle they give you, and subtract it from 90. ( 90 - 45 ) and you have solved what the missing number ( ? ) is. Hope I helped!