The metal spheres are charged to $\begin{array}{c}+\\ -\end{array}300$. Draw this figure on your paper, then draw a plausible contour map of the potential, showing and labeling the equipotential surfaces.

We have given that the metal spheres are charged to $\begin{array}{c}+\\ -\end{array}300$.

The zero potential (the mean of the two potentials) surface will be in the points that are equidistant to the charges creating this potential field. This surface is the plane, perpendicular to the line connecting the two spheres.

The other equipotential surfaces would have been, in the absence of a second charge, spherical shells. Due to this second charge, these surfaces will be paraboloid shells, which will be given as parts of a parabola.

Surely enough, the drawing will be symmetric. Having said all the above, the following would be a sketch of the required equipotential surfaces.

The$0V$surface will be represented by a perpendicular line at the midpoint of the line connecting the two charges, while the other surfaces will be represented by parabolas along the horizontal axis, which are symmetric to each other.