So there's a little acronym in trig that i remember, and its SOH CAH TOA.
The first letter of the 3 words is either cosine, sine, or tangent.
The second letter is the numerator, and the third letter is the denominator. (O=opposite line of angle, H=hypotenuse, A=adjacent line to angle)
Since we are finding the cosine, we are using the acronym CAH, and its going to be
, which in this case is
, or C.