Cosine is adjacent/hypotenuse. Draw a right triangle and you can see that the hypotenuse is 5, one of the legs is 4, and the missing leg would be 3 (3,4,5 Pythagorean triangle).
Now that you have all the sides, use the same angle to find all the other trig functions. sine = opposite/hypotenuse = 3/5 cotangent = adjacent/opposite = 4/3 cosecant = hypotenuse/opposite = 5/3 secant = hypotenuse/adjacent = 5/4
