charges q and –3q are placed fixed on x-axis separated by distance ‘d’. Where should a third charge 2q be placed such that it will not experience any force?

Distance between -3q and q is =d

Let us consider

Distance between q and 2q is = x

Then 2q must be placed at a distance =(d+x)

Now we have to find out the x

From Coulomb’s law

The force exerted by one charge on another charge is

From -3q to 2q is

From q to 2q is

Compare (1) and (2)

(Or)