Verified answer

The electric potential at a distance (r) away from a point charge, q, can be found as,

V = k_e * q / r

Where k_e = 1 / (4 * pi * ε_0) ≈ 8.99 E9 N (m / C)^2

You are told that r = .3 meters and q = 3 E-6 C.

Plugging into the above equation we get,

V = (8.99 E9 N m^2 / C^2) * (3 E-6 C) / (.3 m)

V = 89.9 kN / C = about 90 kV

define electric potential: V = kq/r; you have k = 9.8x10^9, q=charge you got, r = 0.30; the mass doesn't matter unless you're solving for velocity here.