The separation of the plates can be calculated using $E = -Delta V/d$. Solving for $d$ and substituting $E = 2.0times10^{3}$ V/m and $Delta V = -12$ V (from Guided Example 20.7), we find

$$
d = -frac{Delta V}{E} = -frac{-12;mathrm{V}}{2.0times10^{3};mathrm{V/m}} = 0.6;mathrm{cm}
$$

The potential difference between the plates can be calculated using the relationship $E = -Delta V/d$. Since $E = 1.2times10^5$ V/m and $d = 0.75$ mm, we have

$$
Delta V = -Ed = -(1.2times10^5;mathrm{V/m})(0.75times10^{-3};mathrm{m}) = 90;mathrm{V}
$$