To find diagonal $ d2 $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $A = 300\, \text{cm}$ and $d_1 = 15\, \text{cm}$ we have:
$$ 300\, \text{cm} = \dfrac{ 15\, \text{cm} \cdot d_2 }{ 2 } $$$$ 300\, \text{cm} \cdot 2 = 15\, \text{cm} \cdot d_2 $$$$ 600\, \text{cm} = 15\, \text{cm} \cdot d_2 $$$$ d_2 = \dfrac{ 600\, \text{cm} }{ 15\, \text{cm} } $$$$ d_2 = 40 $$