Answer
$2.2\times10^{-18}\ m$
Work Step by Step
We know that the time taken by the electron to travel a distance d is $t=6.72\times 10^{-10}\ s$.
During this time, the electron falls a vertical distance.
So, using kinematics formula;
$y = ut+\frac{1}{2}at^2$
$y=0+\frac{1}{2}gt^2$
$y=0.5\times9.8\times(6.72\times 10^{-10}\ s)^2$
$y=2.2\times10^{-18}\ m$
