To find side $ a $ use formula:
$$ A = \frac{ a^2 \sqrt{3}}{2} + 3 a l $$After substituting $ A = 200 $ and $ l = 2.5 $ we have:
$$ 200 = \frac{ a^2 \sqrt{3}}{2} + 3 \cdot a \cdot 2.5 $$$$ 400 = a^2 \sqrt{3} + 1200 \cdot a $$$$ a^2 \sqrt{3} + 1200 \cdot a - 400 = 0 $$( use quadratic equation solver to solve this quadratic equation )
$$ a = 0.3332 $$