Posted by I need an answer now on .
A 5.0*10^6C point charge is placed at the 0.00 cm mark of a meter stick and a 4.0 *10^6C point charge is placed at the 50 cm mark. At what point on a line joining the two charges is the electric field due to these charges equal to zero?

Physics 
drwls,
Write an equation for the Efield along the line joining the two charges. Let the position of the charge be x. Set the Efield equal to zero and solve for x. You can assume that x has to be > 50 cm, so that the fields due to the two charges oppose each other.
E = k*[(5.0*10^6/x^2) 4*10^6/(x0.5^2]
= 0
k is the Coulomb constant, which you don't need to use since it cancels out.