We use Cosine Law to find the one side of a triangle if there are only 2 sides and 1 irrelevant angle given, if there are 3 sides given we can find an angle too.
For example, to find the a side, we square it and that basically equals the addition of the square roots of other sides, and then we multiply the sides and negative 2 and after that we find the cos of angle A and multiply it with that too. If we want to find the angle, we divide the equation to 2 parts. We first add the square roots of the sides and then subtract it from the square root of the side actual side and then we multiply -2bc and cosA , we divide the side by -2bc and then reverse the cos to cos-1 use it on the side, and after that we find the angle.