Answer:
It would have to be in between the two charges and be positive for force on 9e to be zero
k 9e q/x^2= k 36e 9e /d^2
q/x^2 = 36e/d^2
q = 36e x^2/d^2
Now we need force on 36e to be zero
k 36e q/(d-x)^2 = k 36e 9e/d^2
q/(d-x)^2 = 9e/d^2
36 x^2/d^2 = 9/d^2 (d-x)^2
36 x^2 = 9 (d-x)^2
x=d/3
q = 36e (d/3)^2/d^2 = 36e/3^2 = 4 e