Answer
$1.9\times 10^{-2}\ m^2$
Work Step by Step
The formula for the capacitance with plate area A is given by;
$C =\frac{ A \epsilon_o}{b-a}$
Rearranging the above formula for A,
Area $A = \frac{C(b-a)}{\epsilon_o}$
Now substitute corresponding values i,e
$C = 84.5 pF , a = 0.038 m , b=0.04 m$
$A = \frac{(84.5\times 10^{-12} F)(0.04 m-0.038 m)}{(8.85\times 10^{-12}\ c^2/N.m^2)}$
$A = 1.9\times 10^{-2} m^2$
