To find area $ A $ use formula:
$$ A = \frac{ a^2 \sqrt{3}}{2} + 3 a l $$After substituting $ a = 20 $ and $ l = 15 $ we have:
$$ A = \frac{ 20^2 \sqrt{3}}{2} + 3 \cdot 20 \cdot 15 $$$$ A = \frac{ 400 \sqrt{3}}{2} + 3 \cdot 300 $$$$ A = \frac{ 400 \sqrt{ 3 }}{2} + 900 $$$$ A = 200 \sqrt{ 3 } + 900 $$$$ A = 1246.4102 $$