Answer
$5.8cm$
Work Step by Step
$V=K\frac{q}{r}$ simplifies to $r=\frac{Kq}{V}$.
We plug in the known values in the formula to obtain:
$r=\frac{9\times 10^9(3\times 10^{-8})}{1800-500}$
$r=0.208m=20.8cm$
Thus the distance from the sphere surface$ =20.8-15=5.8cm$
