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 = 4 $ and $ a = 12 $ we have:
$$ 4^2 + d_2^2 = 4 \cdot 12^2 $$ $$ 16 + d_2^2 = 576 $$ $$ d_2^2 = 576 - 16 $$ $$ d_2^2 = 560 $$ $$ d_2 = \sqrt{ 560 } $$$$ d_2 = 4 \sqrt{ 35 } $$STEP 2: find area $ A $
To find area $ A $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $ d_1 = 4 $ and $ d_2 = 4 \sqrt{ 35 } $ we have:
$$ A = \dfrac{ 4 \cdot 4 \sqrt{ 35 } }{ 2 }$$$$ A = \dfrac{ 16 \sqrt{ 35 } }{ 2 } $$$$ A = 8 \sqrt{ 35 } $$