This is classic right triangle trig. Side a, our unknown, is adjacent to angle C which is a given. You always have to use a given angle and side to find a missing side, so we have to figure out how the two givens are related to our unknown. Side a is adjacent to angle C, and we are also given the hypotenuse. We know side a is not the hypotenuse since the hypotenuse is ALWAYS the side across from the 90 degree angle. The tan ratio of angle C will give us a way to have only one unknown.
