If the midpoint of AB is (1,3). and the coordinates of A are (4,1), the coordinates of B can be found by creating an equation using the midpoint formula M=((x1+x2)/2 , (y1+y2)/2)
B's x value can be found using the equation 1 = (4+x)/2 by setting the x value of the midpoint equal to the x value portion of the formula with A's value substituted.
1 = (4+x)/2
2 = 4+x
-2 = x
B's y value can be found using the same method, but using y values from the points. 3 = (1+y)/2
6 = 1+y
5 = y
B's coordinates are (-2,5)