Answer
53 is the angle
Further explanation
Complementary angles sum to 90°.
We can build an equation to find any two complementary angles.
x + y = 90
Either x or y is given. Notice how they're interchangeable which means it doesn't make any difference what value you plug in for x or y.
Allow me to demonstrate.
In our case we have an angle that measures 37.
We can plug in 37 for x or y, but I always plug it in for x.
37 + y = 90
Subtract 37 on each side
y = 53°