We can find the potential using the equation
V = kQ/r
Where V is the potential, k corresponds to the Coulomb constant
k = 9. 10^9 N N. m^2/C^2 ,Q is the charge and r is the radius of the sphere (r = 2.5.10^-3 m)
Substituting in the equation we obtain
V = (9. 10^9 N N. m^2/C^2)*(40.10^-12 C)/(2.5.10^-3 m) =
V = 144 V (Potential near its surface)