To find side $ a $ use formula:
$$ A = a \cdot b $$After substituting $A = 107.52\, \text{cm}$ and $b = 9.6\, \text{cm}$ we have:
$$ 107.52\, \text{cm} = 9.6\, \text{cm} \cdot a $$$$ a = \dfrac{ 107.52\, \text{cm}}{ 9.6\, \text{cm} } $$$$ a \approx 3.5651 $$