Suppose that three astronomical objects

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Homework Statement

Suppose that three astronomical objects (1, 2, and 3) are observed to lie on a line, and that the distance from object 1 to object 3 is D. Given that object 1 has 1.5 times the mass of object 3 and seven times the mass of object 2, find the distance (in terms of D) between objects 1 and 2 for which the net force on object 2 is zero.
____ D m

Homework Equations

F = (G x m1 x m2) / r^2

The Attempt at a Solution

I drew a diagram of this, and had no luck figuring out what to do next.

 

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Does this help?