Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
The electric field at point P that is at $r$ distance from a 15 nC-charge is about 54 kN/C. Find $r$.
$$\color{blue}{\bf [b]}$$
Solving the given formula for $r$,
$$r=\sqrt{\dfrac{(9\times 10^9)(15\times 10^{-9})}{54\times 10^3}}=\bf 0.05\;\rm m$$
$$r=\color{red}{\bf 5}\;\rm cm$$
