To find diagonal $ d $ use Pythagorean Theorem:
$$ a^2 + b^2 = d^2 $$After substituting $a = 0.054\, \text{cm}$ and $b = 0.054\, \text{cm}$ we have:
$$ \left( 0.054\, \text{cm} \right)^{2} + \left( 0.054\, \text{cm} \right)^{2} = d^2 $$ $$ 0.0029\, \text{cm}^2 + 0.0029\, \text{cm}^2 = d^2 $$ $$ d^2 = 0.0058\, \text{cm}^2 $$ $$ d = \sqrt{ 0.0058\, \text{cm}^2 } $$$$ d = 0.0764\, \text{cm} $$