Verified answer

The electric potential from a point charge is q / (4πer), where r is the distance from the charge, and e is the permitivity of the space around the charge (e = ke0, where k = dielectric constant and e0 permitivity of vacuum). You can add the potential contribution from each charge directly (potential is a scalar quantity). Do not forget to include the sign of the charge.

NOTE: the potential is not the same as the potential energy; your question asks for electric potential, but the title refers to potential energy. If you place a charge of value q in an electric field, the potential energy of that charge is q*V, where V is the potential of the field at the location of the charge.

Let q1 be the first charge. The force experienced by a 1C charge (potential)at a distance r1 from it is (q1/4*pi*epsilon*r1^2). (columbs law).Similarly the force due to q2 at a distance r2=(q2/4*pi*epsilon*r^2). The net force is the vector sum of these two. From this expression you can calculate the work done . The answer is

q1/(4*pi*epsilon*0.5*r12) + q2/(4*pi*epsilon*0.5*r12).

where= 1/(4*pi*epsilon)=9 X 10^9. You need to multiply by a charge whose potential energy you want to find.