Scalar Potential

Consider a cube of edge a. There is a point charge q at each corner. Find $$\phi$$ at the center of the face for which x=a.
The answer to the problem is $$\0.707*(q/\epsilon_0*a)$$
I have to use the the scalar potential equation, but I have been stuck on this problem for a while. I know that I have to consider the angle which is cos45 = 0.707. I am not sure I understand what they are asking in the problem.
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 Consider the symmetry of the cube, and remember that the potential is the sum of the potential of each charge. There are 8 charges.
 Recognitions: Homework Help the "1 spot" on a dice cube ... each charge contributes its own portion to the potential there, since it is not the same distance from each source.

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