cosθ is 3/5 because the cosine is found from the adjacent side of the triangle being divided by the hypotenuse. The sine is found by putting the opposite side from θ over the hypotenuse. If you draw this triangle out, the side across from θ would be 4 and the hypotenuse 5. In order to find the opposite side, you could either solve with the pythagorean theorem (a^2+b^2= c^2) or you could remember the properties of a triangle, therefore the final side is 3. You then put the opposite, 3, over the hypotenuse,5, in order to get 3/5 for the cosθ.