To find diagonal $ d2 $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $ A = 300 $ and $ d_1 = 30 $ we have:
$$ 300 = \dfrac{ 30 \cdot d_2 }{ 2 } $$$$ 300 \cdot 2 = 30 \cdot d_2 $$$$ 600 = 30 \cdot d_2 $$$$ d_2 = \dfrac{ 600 }{ 30 } $$$$ d_2 = 20 $$