If "a" and "b" are two values of x-coordinate, and "m" is the midpoint between them, it means the distance from one end to the midpoint is the same as the distance from the midpoint to the other end
... a-m = m-b
When we add m+b to this equation, we get
... a+b = 2m
Solving for m gives
... m = (a+b)/2
This applies to y-coordinates as well. So ...
... The midpoint between (x1, y1) and (x2, y2) is ((x1+x2)/2, (y1+y2)/2)
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Jennifer had (x1, y1) = (-4, 10) and (x2, y2) = (-2, 6). So her calculation would be
... midpoint = ((-4-2)/2, (10+6)/2) = (-6/2, 16/2) = (-3, 8)
Brandon had (x1, y1) = (9, -4) and (x2, y2) = (-12, 8). So his calculation would be
... midpoint = ((9-12)/2, (-4+8)/2) = (-3/2, 4/2) = (-1.5, 2)