The formula of the Pythogorean Theorem is SQRT(a^2 + b^2) = c.
As each side of a square are the same lengths (i.e., a = b), the equation for the diagonal of length 14 inches can be written as
SQRT(a^2 + a^2) = 14
SQRT(2*a^2) = 14
a*(SQRT(2) = 14
a = 14/(SQRT(2).
It is common practice in simplification to place the radical on top. This is accomplished by multiplying the denominator and numerator by 1 in the form of the radical element divided by itself, e.g., SQRT(2) / SQRT(2).
This gives
a = 14*SQRT(2) / SQRT(2)*SQRT(2)
a = 14*SQRT(2) / 2
a = 7*SQRT(2)