A 5.7C charge and a -3.5C charge are placed 25 cm apart. Where can a third charge be placed to the it experiences no net force?

Use

F = \frac{QQ'}{4\pi \epsilon_o r^2}

In each case, the force should be equal. The charges are different though. Now, you should be able to find the distance 'r'. Post what you get! :)