The distance of any two points (x,y) and (a,b) on the plane is given by the formula sqrt[(x-a)^2+(y-b)^2]. In our case we have that the distance between the point P and Q is sqrt[(11-18)^2+(-11+8)^2]=sqrt[49+9]=sqrt(58). In addition, the midpoint between any two points (x,y) and (a,b) on the plane has coordinates ((x+a)/2,(y+b)/2) which in our case implies that the coordinates of the point M are equal to ((11+18)/2,(-11-8)/2)=(29/2,-19/2).
