To find perimeter $ P $ use formula:
$$ P = 2 a + 2 b $$After substituting $a = 12\, \text{cm}$ and $b = 11\, \text{cm}$ we have:
$$ P = 2 \cdot 12\, \text{cm} + 2 \cdot 11\, \text{cm} $$ $$ P = 24\, \text{cm} + 22\, \text{cm} $$ $$ P = 46\, \text{cm} $$