To find side $ a $ use formula:
$$ A = \frac{ a^2 \sqrt{3}}{2} + 3 a l $$After substituting $A = 9\, \text{cm}$ and $l = 14\, \text{cm}$ we have:
$$ 9\, \text{cm} = \frac{ a^2 \sqrt{3}}{2} + 3 \cdot a \cdot 14\, \text{cm} $$$$ 18\, \text{cm} = a^2 \sqrt{3} + 54\, \text{cm} \cdot a $$$$ a^2 \sqrt{3} + 54\, \text{cm} \cdot a - 18\, \text{cm} = 0 $$( use quadratic equation solver to solve this quadratic equation )
$$ a = 0.3298 $$