To find diagonal $ d $ use Pythagorean Theorem:
$$ a^2 + b^2 = d^2 $$After substituting $a = 6.5\, \text{cm}$ and $b = 6\, \text{cm}$ we have:
$$ \left( 6.5\, \text{cm} \right)^{2} + \left( 6\, \text{cm} \right)^{2} = d^2 $$ $$ 42.25\, \text{cm}^2 + 36\, \text{cm}^2 = d^2 $$ $$ d^2 = 78.25\, \text{cm}^2 $$ $$ d = \sqrt{ 78.25\, \text{cm}^2 } $$$$ d = 8.8459\, \text{cm} $$