Answer
$84.85\ \ \mathrm{ ft}$
Work Step by Step
The diagonal of the square is the hypotenuse of a right triangle whose legs are sides of the square.
Apply the Pythagorean theorem.
\begin{aligned}60^{2}+60^{2}&=d^{2}\\
2\cdot 60^{2}&=d^{2}\\\\
d&=\sqrt{2\cdot 60^{2}}\\
&=60\sqrt{2}\\
&\approx 84.85\
\mathrm{ft}\end{aligned}