The diagonal is the hypotenuse of a right triangle with the two legs of x and 24/x.
(√5)2 = x2 + (24/x)2 square terms
5 = x2 + 576/x2 multiply both sides by x2
5x2 = x4 + 576 subtract 5x2 form both sides
0 = x4 - 5x2 + 576
This equation does not have any real roots. Please check your numbers.