To find diagonal $ d $ use Pythagorean Theorem:
$$ a^2 + b^2 = d^2 $$After substituting $a = 2.5\, \text{cm}$ and $b = 2\, \text{cm}$ we have:
$$ \left( 2.5\, \text{cm} \right)^{2} + \left( 2\, \text{cm} \right)^{2} = d^2 $$ $$ 6.25\, \text{cm}^2 + 4\, \text{cm}^2 = d^2 $$ $$ d^2 = 10.25\, \text{cm}^2 $$ $$ d = \sqrt{ 10.25\, \text{cm}^2 } $$$$ d = 3.2016\, \text{cm} $$