Actually, you want to find the position vector of the point M. Since what you give uses p and q for the position vectors of point P and Q, I will call the vector you want m.
M is the mid-point of line segment between P and Q. You are aware, are you not, that if P had coordinates [itex](p_x, p_y)[/itex] and Q had coordinates [itex](q_x, q_y)[/itex], then M would have coordinates [itex]((p_x+ q_x)/2, (p_y+q_y))[/itex]? What does that tell you about m "as a function of p and q"?