Electrostatic force and magnitude problem

[loganblacke]

In the figure, the net electrostatic force on Q1 is zero. If Q1 = +4.99 nC, determine the magnitude of Qo.


Qo = (r0/r1)^2 * abs(q)*2cos theta

The figure is attached, please help, I'm so lost with physics II

 

[tiny-tim]

hi loganblacke!

(have a theta: θ and try using the X² icon just above the Reply box )

forces are vectors, so you have to add them as vectors

start by writing out the three forces as three vectors ​

 

[loganblacke]

How do i determine the direction of the vectors?

 

[tiny-tim]

each vector will point along the line between the two charges