Question

two 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?

Answer 1

F = k qQ/ r^2

put q at the origin and -3q to the left, x1<0

put 2q to the right at a value of x2>0 to that force of (q) on (2q) balances
the force of attraction of (-3q) on (2q)

use the equation to find the distances x1 and x2

Answer 2

now a quadratic eq is formed which has no real roots...

Answer 3

That suggests there may be a sign error.
Of course, I could be wrong. I have not worked out the problem myself.