STEP 1: find side $ a $
To find side $ a $ use formula:
$$ d = 2 \cdot a $$After substituting $d = 30\, \text{cm}$ we have:
$$ 30\, \text{cm} = 2 \cdot a $$ $$ a = \dfrac{ 30\, \text{cm} }{ 2 } $$ $$ a = 15\, \text{cm} $$STEP 2: find perimeter $ P $
To find perimeter $ P $ use formula:
$$ P = 6 \cdot a $$After substituting $a = 15\, \text{cm}$ we have:
$$ P = 6 \cdot 15\, \text{cm} $$ $$ P = 90\, \text{cm} $$