Two point charges of magnitude 7.00C and -9.00C are placed along the x axis at 0 cm and 40.0 cm. Where must a third charge, q, be placed along the x axis so that it does not experience any net force because of the other two charges? a. 0.187 m
b. -0.187 m
c. 0.200 m
d. -2.99 m
e. 2.99 m

Aresistor with an unknown resistance is connected in parallel to a 9 ω resistor. when both resistors are connected in parallel to an emf source of 21 v, the current through the unknown resistor is measured with an ammeter to be 4 a. what is the resistance of the unknown resistor? answer in units of ω.