To find diagonal $ d $ use Pythagorean Theorem:
$$ a^2 + b^2 = d^2 $$After substituting $a = 55\, \text{cm}$ and $b = 33\, \text{cm}$ we have:
$$ \left( 55\, \text{cm} \right)^{2} + \left( 33\, \text{cm} \right)^{2} = d^2 $$ $$ 3025\, \text{cm}^2 + 1089\, \text{cm}^2 = d^2 $$ $$ d^2 = 4114\, \text{cm}^2 $$ $$ d = \sqrt{ 4114\, \text{cm}^2 } $$$$ d = 11 \sqrt{ 34 }\, \text{cm} $$