Use the Pythagorean theorem. If you know the side of one side you know it’s the same as the other side.
Ex) lets say I have a square and one of the sides is 7 the other side has to be 7 because all 4 sides of a square is the same, so use the Pythagorean theorem to find the diagonal.
7 squared + 7 squared = c squared 49 + 49 is 98 so take the square root of that and get about 9.9