You should specify what language you're using in these types of questions; here's an example in C++.
#include <iostream>
#include <math>
int main()
{
// example area
double area = 25;
// square root the area to find the length
// then apply basic pythagoras
double diagonal = sqrt(pow(sqrt(area), 2) + pow(sqrt(area), 2));
return 0;
}
