STEP 1: find side $ a $
To find side $ a $ use formula:
$$ d = 2 \cdot a $$After substituting $d = 40\, \text{cm}$ we have:
$$ 40\, \text{cm} = 2 \cdot a $$ $$ a = \dfrac{ 40\, \text{cm} }{ 2 } $$ $$ a = 20\, \text{cm} $$STEP 2: find diagonal $ s $
To find diagonal $ s $ use formula:
$$ s = \sqrt{ 3 } \cdot a $$After substituting $a = 20\, \text{cm}$ we have:
$$ s = \sqrt{ 3 } \cdot 20\, \text{cm} $$ $$ s = 20 \sqrt{ 3 }\, \text{cm} $$