To find diagonal $ d $ use Pythagorean Theorem:
$$ a^2 + b^2 = d^2 $$After substituting $a = 210\, \text{cm}$ and $b = 314\, \text{cm}$ we have:
$$ \left( 210\, \text{cm} \right)^{2} + \left( 314\, \text{cm} \right)^{2} = d^2 $$ $$ 44100\, \text{cm}^2 + 98596\, \text{cm}^2 = d^2 $$ $$ d^2 = 142696\, \text{cm}^2 $$ $$ d = \sqrt{ 142696\, \text{cm}^2 } $$$$ d = 2 \sqrt{ 35674 }\, \text{cm} $$