STEP 1: find side $ a $

To find side $ a $ use formula:

$$ h = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$

After substituting $h = 3\, \text{cm}$ we have:

$$ 3\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 3\, \text{cm} \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 6\, \text{cm} $$ $$ a = \dfrac{ 6\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a = 2 \sqrt{ 3 }\, \text{cm} $$

STEP 2: find area $ A $

To find area $ A $ use formula:

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

After substituting $a = 2 \sqrt{ 3 }\, \text{cm}$ and $l = 10\, \text{cm}$ we have:

$$ A = \frac{ 2 \sqrt{ 3 }\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 2 \sqrt{ 3 }\, \text{cm} \cdot 10\, \text{cm} $$$$ A = \frac{ 12\, \text{cm}^2 \sqrt{3}}{2} + 3 \cdot 20 \sqrt{ 3 }\, \text{cm}^2 $$$$ A = \frac{ 12 \sqrt{ 3 }\, \text{cm}^2}{2} + 60 \sqrt{ 3 }\, \text{cm}^2 $$$$ A = 6 \sqrt{ 3 }\, \text{cm}^2 + 60 \sqrt{ 3 }\, \text{cm}^2 $$$$ A = 66 \sqrt{ 3 }\, \text{cm}^2 $$

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