Two charges are located on the x axis as shown in the figure. d=10.0 cm, q​1​​ =+6.97 μC, and q​2​​ =−10.4 μC. What charge q​3​​ needs to be placed at position x=3d so that the magnitude of the electric field at the origin is zero?

Answer 1

you want
3
∑q_iq_j/d_ij^2 = 0
i,j=1
i≠j
for suitable definitions of d_ij, which you have not provided

Answer 2

In other words we do not have your figure so we do not really know where the first two charges are located.