If two angles are <u>complementary</u>,
then the sum of their measures is 90 degrees.
If two numbers add to 90, we can represent those numbers as <em>x</em> and <em>90 - x</em>.
So we will call one of our angles x and its complement as 90 - x.
So we have 90 - 12 which is 78.
So the complement of a 12° angle is a 78° angle.