Solve Electric Field Problem with Two Point Charges

[Ryo124]

Can someone please walk me through this problem and help me solve it?

Two point charges, q1 = 4.0×10-6 C and q2 = -1.0×10-6 C, are located on the x-axis at
x1 = -1.0 cm and x2 = 3.0 cm.

(a) Determine the electric field at the origin.

(b) Determine the x coordinate of a point on the positive x-axis where the electric field is zero; i.e., a test charge placed at this point would experience no force.

 

[Ryo124]

Alright, I solved the problem for a). Now I am wondering how to approach part b). I really don't have a clue on where to start other than I know that the forces on the test charge are equal and opposite.