If u know the side length of a square how can you find its diagonal explain?

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