Since it is a right angled triangle you can use the Pythagorean Theorem, a^2 + b^2 = c^2, to solve for the hypotenuse, the diagonal.
7^2 + 7^2 = c^2
49 + 49 = c^2
98 = c^2
the square root of 98 = 7 and the square root of 2.
So you're answer is shister F.
