we know that
\(Perimeter\ of\ a\ square=4b\)
where
b is the length side of a square
Step \(1\)
Find the length side b of a square
\(P=56cm\\ P=4b\\ 56=4b\\ b=56/4\\ b=14cm\)
Step \(2\)
Find the length of the diagonal of a square
Applying the Pythagorean Theorem
\(D=\sqrt{b^{2}+b^{2}} \\ D=\sqrt{14^{2}+14^{2}}\\ D=14\sqrt{2} cm\\ D=19.80cm\)
therefore
the answer is
\(19.80cm\)
