To find side $ b $ use formula:
$$ P = 2 a + 2 b $$After substituting $P = 22.4\, \text{cm}$ and $a = 8.3\, \text{cm}$ we have:
$$ 22.4\, \text{cm} = 2 \cdot 8.3\, \text{cm} + 2 b $$ $$ 22.4\, \text{cm} = 16.6\, \text{cm} + 2 b $$ $$ 2 b = 22.4\, \text{cm} - 16.6\, \text{cm} $$ $$ 2 b = 5.8\, \text{cm} $$ $$ b = \dfrac{ 5.8\, \text{cm} }{ 2 } $$ $$ b = 2.9\, \text{cm} $$