I don't really know where to start on the problem. Would I use V = k(q/r)? But what would I do afterwards.

Question

Much explanation on how to go about the problem is much appreciated. Thank you.

1) Three point charges are arranged at the corner of a square of side l. What is the potential at the fourth square (point A)?

At the left upper corner of the square is +Q. at the left lower corner is +3Q, at the right upper corner is -2Q, at the right lower corner is A. All three sides have l.

Answer 1

potential are acalar, not vectors, so you juast add them.
Vtotal=V1+V2+V2
Vtotal=k(q1/d1 + Q2/d2 + q3/d3)

where q1, q2, q3 are the charges, and d1, d2, d3 are the distances to those charges.