$x_2 = \frac{X P_1}{P_1 + P_2}$
$x_1 =\frac{X P_2}{P_1 + P_2}$
$V_0 =\frac{1}{2 \epsilon}[P_2 x_2 x_1+P_2 x_2^2]$
$=\frac{1}{2 \epsilon} P_2 x_2(x_1+x_2)=\frac{1}{2 \epsilon} P_2 x_2 X$
$V_0=\frac{1}{2 \epsilon} \frac{P_2 X^2 P_1}{P_1+P_2} $
$ X^2= 2 \epsilon V_0 \frac{P_1+P_2}{P_1 P_2}$
$X=\sqrt{2 \epsilon_0 V_0 (\frac{1}{P_1}+\frac{1}{P_2})}\qquad$
$\qquad X=\sqrt{\frac{2 \epsilon V_0}{e} (\frac{1}{N_A}+\frac{1}{N_D})}$