Answer
$V=6.4\times10^8V$
Work Step by Step
We know that
$E=K\frac{Q}{R^2}$
The above formula can be written as;
$Q=\frac{ER^2}{K}$ where $R=6400\times 10^3m$. $R$ is the radius of the Earth.
We plug in the known values to obtain:
$Q=\frac{100\times (6400\times 10^3)^2}{9\times 10^9}$
$Q=4.5511\times 10^5C$
Now we can determine electric potential as;
$V=K\frac{Q}{R}$
We plug in the known values to obtain:
$V=9\times10^9\times \frac{4.5511\times10^5}{6400\times 10^3}$
$V=6.4\times10^8V$