Find the magnitude of the electrostatic force

[DeadxBunny]

I am having difficulties with the following problem. I have gotten parts (a) and (b) (my correct answers are shown below), but I cannot figure out how to do part (c). Any help with it would be greatly appreciated. Thanks!
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The charges and coordinates of two charged particles held fixed in an xy plane are q1 = +3.5 µC, x1 = 4.0 cm, y1 = 0.50 cm, and q2 = -4.0 µC, x2 = -2.0 cm, y2 = 1.5 cm.
(a) Find the magnitude of the electrostatic force on q2.
Answer: 34.05N
(b) Find the direction of this force.
Answer: 350.54° (counterclockwise from the +x axis)
(c) At what coordinates should a third charge q3 = +5.0 µC be placed such that the net electrostatic force on particle 3 due to particles 1 and 2 is zero?

 

[stunner5000pt]

first of all it will help if you drew a diagram
(a rough sketch of the xy plane and plot these points)

now the force of Q1 on Q3 shouldbe equal to the force of Q1 on Q2.
(theres many ways to do this, here's on of them)

consider the X coordinate system ONLY
the distance between Q1 and Q2 is 6 units. If a charge Q3 were to be placed such taht F (of Q1 on Q3) = F(of Q3 on Q2) then it would be placed along the line joining Q1 and A3. If Q3 were placed x units from Q1 how was is placed from Q2? Now using F (of Q1 on Q3) = F(of Q3 on Q2) and you know how far Q2 is from Q1 and Q3, you can solve for x.
DO the similar thing for the Y direction. Thus you have your answer