So, the formula for finding the midpoint of a segment given the coordinates of its endpoints is:
((x1+ x2)/2, (y1 + y2)/2 {Average of the x coordinates, average of the y coordinates}.
For this problem we know the coordinates of the midpoint along with one of the endpoints.
So the x coordinate of the midpoint (9) must equal (12 + x2)/2.
That is: 9 = (12 + x2)/2 Solve for x2. Follow a similar pattern to find the y coordinate of the unknown endpoint.