(1) 2 particles of equal mass are fixed at X=0 and another +ve point on the x axis.

No other grav influences are in the system. (they can be ignored)

I have to derive an expression for Vgrav at a general position x, on the x axis.

I believe this is related to V=-GM/r, but can't see how to develop it for 2 masses.

(2) I have to say what the significance of the point where dV/dx=0 is, and what it's coords are in the system.

i believe the derivative is where there is no slope, so no V. Am I right?

I believe the point is on the x axis at infinity, and y=0

Any steers appreciated. Thanks