Answer
0.062 m
Work Step by Step
Given/known: $C=50\,pF,$ $\kappa=1$ (for air), $A=0.35\,m^{2}$ and $\varepsilon_{0}=8.854\times10^{-12}\,F/m$
Recall: $C=\frac{\kappa\varepsilon_{0}A}{d}$ or $d=\frac{\kappa\varepsilon_{0}A}{C}$
Result: $d=\frac{1\times8.854\times10^{-12}\,F/m\times0.35\,m^{2}}{50\times10^{-12}\,F}=0.062\,m$
