Force on 3rd Charge w/ 7.98nC: 3.17E-5N

[Digdug12]

Homework Statement

Keeping all other values the same in the problem suppose now that the third charge has a value of 7.98 nano coulombs. What is the magnitude of the total force exerted by the first two charges on the third charge. Give your answer in scientific notation to three significant digits in Newtons.

The previous information was:

A negative charge –0.779 micro coulombs exerts an upward 0.412 N force on an unknown charge 0.300 m directly below it. What is the magnitude of the unknown charge?

I found the magnitude of the unknown charge to be 5.30E-6

Homework Equations

F=kQ₁Q₂/R²

The Attempt at a Solution

I tried using the basic F=kQ₁Q₂/R² equation, substituting in the new charge, and using the one I found in the previous problem, keeping the distance of .300m the same. This didnt work, so I tried to sum the forces from force one->Force Three and Force two-> force three, but the answer came out 1x10^3 to big, and slightly off. What am I doing wrong?

 

[ideasrule]

Where is the third charge with respect to the other two? If this is not specified, the question can't be answered.