To find diagonal $ d2 $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $ A = 35 $ and $ d_1 = 10 $ we have:
$$ 35 = \dfrac{ 10 \cdot d_2 }{ 2 } $$$$ 35 \cdot 2 = 10 \cdot d_2 $$$$ 70 = 10 \cdot d_2 $$$$ d_2 = \dfrac{ 70 }{ 10 } $$$$ d_2 = 7 $$