Answer
The electric potential just outside the surface is $~6.3\times 10^5~V$
Work Step by Step
Outside a spherical conductor, the electric field can be expressed as $E = \frac{k~q}{r^2}$, where $q$ is the total charge on the conductor.
Outside a spherical conductor, the electric potential can be expressed as $V = \frac{k~q}{r}$, where $q$ is the total charge on the conductor.
Note that $V = E~r$. We can find the electric potential just outside the surface:
$V = E~r = (8.40\times 10^5~V/m)(0.75~m) = 6.3\times 10^5~V$
The electric potential just outside the surface is $~6.3\times 10^5~V$
