(x,y) ---> (12,16) so 12 is your x value and 16 is your y value.

Plot this point on a unit circle and connect the center to the point. The angle from the horizontal x axis to the line is θ.

Have you ever learned SOHCAHTOA

since you are dealing with cos, you want to know that cos is adjacent/hypothesis.

In this instant, the hypothesis is the line you drew on the unit circle. To find it, we use the pathagorean theorem a^2 + b^2 = c^2.

12^2 + 16^2 = 400

take the square root to get c, which = 20

now is when we look back to cos = adjacent / hypothesis

the adjacent value is 16, while the hypothesis value is 20.

16/20 is your answer, but when you simplify (both have multiples of 4 so divide both the numerator and denominator by this)

AND AS YOU SAID...

your answer will be 4/5