Right away we know we don't have fractions; the midpoint is the average of the coordinates, so if it and one endpoint are integers so is the other endpoint.
We can do a kind of point arithmetic:
C = (A+B)/2
2C = A+B
B = 2C - A
B = 2(6,1) - (1,3) = (12,2)-(1,3)=(11,-1)
Answer: That's none of the above, but a typo away from the first choice.
