What are the x-coordinates where E=0?

The figure shows a very large nonconducting sheet that has a uniform surface charge density - σ, where σ > 0 denotes the absolute value of the charge density; it also shows a particle of charge Q > 0, at distance d from the sheet. Both are fixed in place. (a) At what positive coordinate on the x axis (other than infinity) is the net electric field of the sheet and particle zero? In order for there to be a negative x coordinate where the field vanishes, the ratio Q/σ must have some minimal value. (b) What is that minimum value, and (c) what is the corresponding x coordinate?

There are no numerical values given to the variables. All I was given are the variables themselves. It must be one of those mathematically conceptual problems. I was able to find the answers to parts A and B, but C seems to be beyond my grasp. Any help?

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