Help with determining distance where electric potential is 0

[Hannah7h]

Question states 'Determine the distance from the +4.0nC charge to the point, along the straight line between the charges, where the electric potential is zero.'

This is as far as I've got: so I've used V=Q/4pi(epsilon0)r and then set it so V1+V2=0 (to find the point where the resultant electric potential is zero). Then the 4pi(epsilon0) can just be ignored as it is a constant, therefore V1=Q1/R1 and then V2=Q2/R2 therefore Q1/R1 + Q2/R2 = 0 so then (substituting in the values), 4x10^-9/r - 8x10^-9/ (80x10^-3) -r = 0 AND now I'm not sure where to go from this point

Thank you for any help

 

[Ssnow]

do the sum between fractions and solve the equation in $r$ ...

 

[Hannah7h]

do the sum between fractions and solve the equation in $r$ ...

yeah that's the bit I can't seem to do my answer keeps coming out at 7000

 

[Hannah7h]

do the sum between fractions and solve the equation in $r$ ...

No worries I've got it now thanks!