please see the attched figure o better understand the problem
Applying the Pythgorean Theorem
d^2=b^2+b^2
we have
b=10 units
substitute
d^2=10^2+10^2
d^2=100+100
d^2=200
[tex]d=\sqrt[]{200}[/tex]
simplify
[tex]d=10\sqrt[\text{ }]{2\text{ }}\text{ units}[/tex]
the diagonal is
[tex]d=10\sqrt[\text{ }]{2\text{ }}\text{ units}[/tex]
