Two angles are complementary if their degree measures add up to 90 degrees. So if the angle is $x$ degrees, then its complement is $90-x$ degrees. We set up the equation $x=2(90-x)$ and solve for $x$. $$x=180-2x\qquad\Rightarrow3x=180\qquad\Rightarrow x=60$$So the angle has a degree measure of $60$ degrees.
What is the degree measure of an angle whose measure is double the measure of its complement?