To find diagonal $ d $ use Pythagorean Theorem:
$$ a^2 + b^2 = d^2 $$After substituting $a = 11\, \text{cm}$ and $b = 18\, \text{cm}$ we have:
$$ \left( 11\, \text{cm} \right)^{2} + \left( 18\, \text{cm} \right)^{2} = d^2 $$ $$ 121\, \text{cm}^2 + 324\, \text{cm}^2 = d^2 $$ $$ d^2 = 445\, \text{cm}^2 $$ $$ d = \sqrt{ 445\, \text{cm}^2 } $$$$ d = \sqrt{ 445 }\, \text{cm} $$