#### Answer

$L = 4.8~cm$

#### Work Step by Step

We can find the value of $L$:
$C = \frac{\epsilon_0~A}{d}$
$C = \frac{\epsilon_0~L^2}{d}$
$L^2 = \frac{C~d}{\epsilon_0}$
$L = \sqrt{\frac{C~d}{\epsilon_0}}$
$L = \sqrt{\frac{(100\times 10^{-12}~F)(0.20\times 10^{-3}~m)}{8.854\times 10^{-12}~F/m}}$
$L = 0.048~m$
$L = 4.8~cm$