To find area $ A $ use formula:

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

After substituting $a = 18\, \text{cm}$ and $l = 20\, \text{cm}$ we have:

$$ A = \frac{ 18\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 18\, \text{cm} \cdot 20\, \text{cm} $$$$ A = \frac{ 324\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 360\, \text{cm}^2 $$$$ A = \frac{ 324 \sqrt{ 3 }\, \text{cm}^2}{2} + 1080\, \text{cm}^2 $$$$ A = 162 \sqrt{ 3 }\, \text{cm}^2 + 1080\, \text{cm}^2 $$$$ A = 1360.5922\, \text{cm}^2 $$

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