As Q3 remains stationary, it is clear that it is not accelerating. Thus we can deduce from Newton's second law that there is no resultant force acting upon the charge. However there are still electrostatic forces acting on the charge from Q1 and Q2. Thus as there is no resultant forces, we can deduce that the force between Q1 and Q3 is equal to the force between Q2 and Q3. Knowing this we can use coulomb's law to equate the two forces and rearrange for the desired ratio. F(1,3) = F(2,3). If we call the distance between Q2 and Q3 d, and the distance between Q1 and Q3 s, then: k*(4q2)/s2 = k*(6q2)/d2 where we are trying to solve for d/s. Rearranging gives d/s = (3)1/2.