Graph the point to form a right triangle. The point lies in quadrant 4. Cosine is positive in quadrant 4.
3^2 + (-4)^2 = c^2
25 = c^2
Taking the square root on both sides leads to c = 5.
c = 5 = hypotenuse
Adjacent = 3
cos θ = adjacent/hypotenuse
cos θ = 3/5
Done!
