Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
Two identical charges of 3 nC are on the same line and the separation distance between them is $d$. If we know that the net electric potential 3 cm away to the right from the right charge is 1200 V, find $d$.
$$\color{blue}{\bf [b]}$$
Solving the given formula for $d$;
$$d=\color{red}{\bf 6}\;\rm cm$$
