The cosine rule is like the sine and tangent. If you are on about trigonometry then first you need to work out the longest side of the triangle-the hypotenuse. Then the side opposite to the angle you are looking for-opposite. and the last side is the adjacent. Ok, so you now have the two sides that were already given to you or the angle or something that you need to work out. In your calculator type in 'cos' and then your number. Lol understand??