To find area $ A $ use formula:
$$ A = \dfrac{ d_1 \cdot d_2 }{ 2 } $$After substituting $d_1 = 3.5\, \text{cm}$ and $d_2 = 0.8\, \text{cm}$ we have:
$$ A = \dfrac{ 3.5\, \text{cm} \cdot 0.8\, \text{cm} }{ 2 }$$$$ A = \dfrac{ 2.8\, \text{cm}^2 }{ 2 } $$$$ A = 1.4\, \text{cm}^2 $$