What are complementary angles?
Complementary angles are simply two angles that form a right angle which are 90 degree angles.
Here, the right angle tells us that the two angles are complementary(right), meaning that the sum of these two angles is 90 degrees.
Solving the question
Because the two angles add up to 90 degrees, we can say that:
a + 56 = 90
==> subtract 56 from both sides
a = 34°
The measure of the missing angle is 34 degrees.