To find area $ A $ use formula:

$$ A = \frac{ a^2 \sqrt{3}}{2} + 3 a l $$

After substituting $a = 20\, \text{cm}$ and $l = 15\, \text{cm}$ we have:

$$ A = \frac{ 20\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 20\, \text{cm} \cdot 15\, \text{cm} $$$$ A = \frac{ 400\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 300\, \text{cm}^2 $$$$ A = \frac{ 400 \sqrt{ 3 }\, \text{cm}^2}{2} + 900\, \text{cm}^2 $$$$ A = 200 \sqrt{ 3 }\, \text{cm}^2 + 900\, \text{cm}^2 $$$$ A = 1246.4102\, \text{cm}^2 $$

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