You can use the midpoint formula, knowing the midpoint between two points (x1,y1) and (x2,y2) has the coordinates
[(x1 + x2)/2, (y1 + y2)/2]
Substituting what we are given, we get:
(2 + x2)/2 = 5 and (5 + y2)/2 = 1
so,
2 + x2 = 10 and 5 + y2 = 2
Solving for x2 and y2 puts the endpoint at (8,-3), which is in the 4th quadrant.
