To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $r = 2.25\, \text{cm}$ we have:
$$ 2.25\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 2.25\, \text{cm} \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 4.5\, \text{cm} $$ $$ a = \dfrac{ 4.5\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a \approx 2.5981\, \text{cm} $$