Square root 156.25 to get the values of the sides of the square. You will get 12.5m for each side.
Now that you have the side values, use Pythagoras' Theorem to solve for the diagonal. Let's call the diagonal x,
c^2 = a^2 + b^2
x^2 = 12.5^2 + 12.5^2
= 312.5
Square root both sides to get x,
x = 17.68m (2 d.p)
HOPE THIS HELPED! Ask me if there is anything you don't understand from this working :D
