Diagonals of a Square

Using pythagorean theorem:  c^2 = a^2 + b^2    where c = diagonal and 'a' and 'b' are the sides...BUT 'a' and 'b' are equal because it is a square:

 

(7 sqrt2)^2 = 2 a^2        Take sqrt of both sides

7sqrt2 = a sqrt2              Divide both sides by sqrt2

7 = a     The sides are 7 inches