Question

The electric potential at a position located a distance of 21.9 mm from a positive point charge of 6.70

Answer 1

You may be knowing that Electric Potential is a scalar quantity. So total potential at a point is the sum of individual potentials at that point. So,
q1 = 6.70 * 10^9 C
q2 = x C
r1= Distance between q1 and the potential point
r2= distance between q2 and potential point

Net Potential = Sum of individual potentials
1.18 * 10^3 = K( q1/r1 + q2/r2).
Use this formula. You have everything except q2. And remember to take distance (r) in metres.
Hope you get the correct answer.