Answer:
cos(A) = 12/13
Explanation:
There are several ways to remember trigonometric functions, but my personal favorite is:
SOH - CAH - TOA
Where Sin(angle) = Opposite/Hypotenuse
Cos(angle) = Adjacent/Hypotenuse
and Tan(angle) = Opposite/Adjacent
The opposite is the side of a triangle that does not form part of the angle. E.g. for angle A the opposite is 5, for angle B the opposite is 12.
The hypotenuse is the longest side of the triangle, or the opposite in relation to the right angle (signified here by the little red box in the bottom left).
The adjacent is the side that isn't the hypotenuse, but is still next to a given angle. E.g. the adjacent for angle A is 12, and the adjacent for angle B is 5.
The cosine of angle A is the adjacent of angle A divided by the hypotenuse. The side adjacent to A is 12, and the hypotenuse is 13. Therefore, the cosine of angle A is 12/13.