How Does Distance Affect Gravitational Force Between Objects?

[physics1234]

Objects with masses of 285 kg and a 585 kg are separated by 0.390 m. Find the net gravitational force exerted by these objects on a 30.0 kg object placed midway between them. Then, at what position (other than infinitely remote ones) can the 30.0 kg object be placed so as to experience a net force of zero?

I found the net gravitational force to be .000015798. The second part of the question is what I don't get. I tried F1-F2=0 and F2-F1=0 and setting them equal to each other so that

.000000451/(.39-x)^2 = .000001491/x^2

but it didn't come out right.

 

[Hootenanny]

Consider the force exerted on the third object by each of the two masses; set the distance from mass 1 to mass 3 as d;

$$F_{13}=F_{23} \Rightarrow \frac{Gm_{1}m_{3}}{d^{2}} = \frac{Gm_{2}m_{3}}{(0.390-d)^2}$$

Does that make sense?