You can work out the diagonal by using Pythagoras theorem. (a^2 = b^2 + c^2)
As it is a square, all 4 sides are the same so it would be:
11.2^2 + 11.2^2 = diagonal^2
250.88 = diagonal^2
Square root both sides to get what the diagonal is equal to.
diagonal is 15.8 cm