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