To find diagonal $ d2 $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $A = 48\, \text{cm}$ and $d_1 = 12\, \text{cm}$ we have:
$$ 48\, \text{cm} = \dfrac{ 12\, \text{cm} \cdot d_2 }{ 2 } $$$$ 48\, \text{cm} \cdot 2 = 12\, \text{cm} \cdot d_2 $$$$ 96\, \text{cm} = 12\, \text{cm} \cdot d_2 $$$$ d_2 = \dfrac{ 96\, \text{cm} }{ 12\, \text{cm} } $$$$ d_2 = 8 $$