Plot your point in an x/y coordinate plane. We are in QII where x is negative and y is positive. From that point, if you drop an altitude to the negative x axis, you have a right triangle with a base measure of -2, a height of 5 and a hypotenuse that is unknown as of right now. We will find it using Pythagorean's Theorem.
The length of the hypotenuse is √29. That means that the cosine of that angle is the side adjacent over the hypotenuse. Rationalizing the denominator gives us