To find area $ A $ use formula:

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

After substituting $a = 8.5\, \text{cm}$ and $l = 12.8\, \text{cm}$ we have:

$$ A = \frac{ 8.5\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 8.5\, \text{cm} \cdot 12.8\, \text{cm} $$$$ A = \frac{ 72.25\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 108.8\, \text{cm}^2 $$$$ A = \frac{ 125.1407\, \text{cm}^2}{2} + 326.4\, \text{cm}^2 $$$$ A = 62.5703\, \text{cm}^2 + 326.4\, \text{cm}^2 $$$$ A = 388.9703\, \text{cm}^2 $$

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