To find diagonal $ d2 $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $A = 600\, \text{cm}$ and $d_1 = 40\, \text{cm}$ we have:
$$ 600\, \text{cm} = \dfrac{ 40\, \text{cm} \cdot d_2 }{ 2 } $$$$ 600\, \text{cm} \cdot 2 = 40\, \text{cm} \cdot d_2 $$$$ 1200\, \text{cm} = 40\, \text{cm} \cdot d_2 $$$$ d_2 = \dfrac{ 1200\, \text{cm} }{ 40\, \text{cm} } $$$$ d_2 = 30 $$