The expression for the magnitude of the electric field between two uniform conducting plates is

Here, V is potential difference between plates and d is separation between plates.
As the potential 6.00 cm from the zero volt plate (and 4.00 cm from the other) is 420 V.
Therefore,

Thus, the electric field strength between the plates is 7000 V/ m
Velocity ratio is also defined as the ratio of a distance through which any part of a machine moves, to that which the driving part moves during the same time. An object has a mechanical advantage if it exerts a force higher than the velocity ratio.
It takes approximately 8 minutes and 20 seconds.
Looks like you simply substitute the length of the femur