The expression for the magnitude of the electric field between two uniform conducting plates is
[tex]E=\frac{V}{d}[/tex]
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,
[tex]E = \frac{420 \ V}{6 \times 10^{-2} m } = 70 \times 10^2 \ V/m[/tex]
Thus, the electric field strength between the plates is 7000 V/ m