Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
We have $N$ electrons that produce a net electric field of $1.5\times 10^6\;\rm N/C$ at a point which is at a distance of $1\;\rm \mu m$ from these electrons. Find how many electrons are placed there.
$$\color{blue}{\bf [b]}$$
Solving the given formula for $N$;
$$N=\dfrac{(1.0\times 10^{-6})^2(1.5\times 10^6)}{(9\times 10^9)(1.6\times 10^{-19})}=\color{red}{\bf 1.0\times 10^3}\;\rm e^-$$
