Answer
$d=90\sqrt{2}\approx127.2792206$
Work Step by Step
As the diagonal of the square is the hypotenuse of a right-angled triangle, the distance can be calculated using the Pythagorean Theorem:
$d^2=a^2+b^2\\
d^2=90^2+90^2\\
d^2=2(90^2)\\
d=\sqrt{2(90)^2}$
$d=90\sqrt{2}\approx127.2792206$
