The diagonal of the square would create a right triangle. With that right triangle we could use Pythagorean’s Theorem to solve for the hypotenuse. Since the legs are given as both 80 then you would set a regular Pythagorean’s Theorem equation (a^2+b^2=c^2) as 80^2+80^2=c^2. Next you would put the squares into regular form and would leave you with 6400+6400=c^2. You then would add them together and find the square root of 12800 (6400 and 6400 added together) after the square root is acquired then you would get c=113.13708 or the diagonal would equal 113.13708. Round as needed.