Finding Equilibrium Charge in a Static System

[LIZard137]

Homework Statement

In the figure below the charge in the middle is Q = -4.6nC. For what charge q1 will charge q2 be in static equilibrium?

q1-----10cm------Q------10cm------q2

I know that the answer is supposed to be 18nC.

Homework Equations

F=K_e lq1llq2l/r²

The Attempt at a Solution

Since the charges are in static equilibrium, I'd assume that F=0. So...
0=(k_el-4.6llq1l/10²)+(k_elq1lq2l/20²)

I've tried several ways of working this problem with Coulomb's Law, but I just can't seem to find the initial step in the right direction.

 

[tiny-tim]

Hi LIZard137!

Just use dimensional analysis …

if you double the distance, what happens to the force? ​

 

[LIZard137]

Wow I completely over thought that... Now I feel retarded lol. Thanks for the help.