STEP 1: find diagonal $ d2 $
To find diagonal $ d2 $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $d_1 = \sqrt{ 6 }\, \text{cm}$ and $a = 4\, \text{cm}$ we have:
$$ \left( \sqrt{ 6 }\, \text{cm} \right)^{2} + d_2^2 = 4 \cdot \left( 4\, \text{cm} \right)^{2} $$ $$ 6\, \text{cm}^2 + d_2^2 = 64\, \text{cm}^2 $$ $$ d_2^2 = 64\, \text{cm}^2 - 6\, \text{cm}^2 $$ $$ d_2^2 = 58\, \text{cm}^2 $$ $$ d_2 = \sqrt{ 58\, \text{cm}^2 } $$$$ d_2 = \sqrt{ 58 }\, \text{cm} $$STEP 2: find area $ A $
To find area $ A $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $d_1 = \sqrt{ 6 }\, \text{cm}$ and $d_2 = \sqrt{ 58 }\, \text{cm}$ we have:
$$ A = \dfrac{ \sqrt{ 6 }\, \text{cm} \cdot \sqrt{ 58 }\, \text{cm} }{ 2 }$$$$ A = \dfrac{ 2 \sqrt{ 87 }\, \text{cm}^2 }{ 2 } $$$$ A = \sqrt{ 87 }\, \text{cm}^2 $$