If the point at which you are calculating the electric field is much farther from the charged solid than the dimensions of the solid itself, the irregularly shaped solid can be approximated as a point charge carrying the same amount of charge as the solid.
The electric field strength is given by:
E = kQ/r²
E = electric field strength, k = Coulomb constant, Q = charge, r = distance from solid
Given values:
k = 9×10⁹Nm²C⁻², Q = 3.0×10⁻⁹C, r = 8.0m
Plug in and solve for E:
E = 9×10⁹(3.0×10⁻⁹)/8.0²
E = 0.42N/C