To find perimeter $ P $ use formula:
$$ P = 2 a + 2 b $$After substituting $a = 3\, \text{cm}$ and $b = 3 \sqrt{ 2 }\, \text{cm}$ we have:
$$ P = 2 \cdot 3\, \text{cm} + 2 \cdot 3 \sqrt{ 2 }\, \text{cm} $$ $$ P = 6\, \text{cm} + 6 \sqrt{ 2 }\, \text{cm} $$ $$ P = 14.4853\, \text{cm} $$