Hint: Use Coulomb's law.
Step 1: Find the location of the third charge.
Here, let us keep the charge 2q at a distance r from A.
Thus, the charge 2q will not experience any force when the force of repulsion on it due to q is balanced by the force of attraction on it due to -3q, at B, where AB = d.
Step 2: Find the net force on the charge and put it equal to zero.
Thus, force of attraction by -3q = Force of repulsion by q
Step 3: Choose the value with a positive sign.
(For negative sign, the position of the charge will be between q and -3q and hence is unadaptable.)
to the left of q.