How long is the diagonal of a 21 ft square?

Well, honey, if you paid attention in geometry class, you'd know that the diagonal of a square can be found using the Pythagorean theorem. So, for a 21 ft square, you take 21 squared plus 21 squared, then take the square root of that sum. In this case, the diagonal would be approximately 29.7 feet. Hope that helps, sugar!