The lowercase sides are opposite to the corresponding uppercase letter. So in the first example, a is 15 because its directly opposite to A. So under the “a^2” section, put 15^2. Do the same for the other sides.
Under a^2 + b^2 __ c^2, use Pythagorean theorem to solve for each equation and see if a^2 + b^2 is greater, less than or equal to c^2 (hypotenuse or LARGEST side of the triangle)