The force between them, together with Coulomb's law, will tell you the value of the product Q1*Q2. Since you know Q1 = -18 already, you can solve for Q2.
It is not clear whether the - sign in front of the force indicates the charges attract or repel.
Consider 2 charged objects. One has a negative charge if 18 colombs. The 1 charges are separated by a distance if 0.9 m and a force of -2.7 N exists between them. Determine the charge of the other object.
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