In free space, a point charge $Q_1=4eC$ is located at $(0,0,0)$ and a point charge $Q_2=-eC$ is located at $(5,0,0).$

a$)$ Find the potential at point $P(3,-5,10)$ due to these two point charges.

$V=,V$

b$)$ It is known that the equipotential surface having $V=0V$ is a spherical surface. Find the location of the origin of the equipotential surface having $V=0V.$

$O=(,$dots$)$

c$)$ Find the radius $R$ of the equipotential surface having $V=0V.$

$R=$