To find the solution to this, we need to use both the Pythagorean Theorem, and trigonometry.
Because we were give that , and knowing that cosine is the ratio of the side adjacent to the hypotenuse, we can draw a triangle and use the information to solve.
First, we need to find the value of the missing side, labeled x in the diagram below:
From the Pythagorean Theorem, we know that , so we can use this to find the missing side:
Now, to find the solution, we use the cosine ratio again, from angle B. We found the measure of the side adjacent to angle B (above) so we can fill in the ratio with the values : . The solution is C.