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