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Now the charges Q1= 1.73x10-6 C and Q2= -2.46 x10-6 C are fixed at their positions, distance 0.257 m apart, and the charge Q3= 3.03 x10-6 C is moved along the straight line. For what position of Q3 relative to Q1 is the net force on Q3 due to Q1 and Q2 zero? Use the plus sign for Q3 to the right of Q1.

and the hint:

Write down an expression for the force on the third charge with the distance from the first charge as a variable. You do not need to solve a quadratic equation.

What I did to try to get the answer was have the the sum of F21 and F23 equal to zero. F23 has the unknown, r, which is the distance from 2 to 3. So I just try to solve for r in order to get the answer, but it is not right.