how to get the answer for this problem
(x1+x2/2,y1+y2/2
looks like the formula for the midpoint of two points (x1,y1) and (x2,y2)
e.g.
if you had the points (1,5) and (-3,7)
then the midpoint, or the result of your expression, would by ((-3+1)/2,7+5)/2) = (-1,6)