What The Equation Of The Perpendicular Bisector Of The Line Segment Whose Endpoints Are (-1,1) And (7,5)?

The midpoint of the segment is
  ((-1+7)/2, (1+5)/2) = (3, 3)
The slope of the given segment is
  (5-1)/(7-(-1)) = 4/8 = 1/2
The slope of the bisector will be
  -1/(1/2) = -2

One way to write the equation you want is
  y = -2(x-3) + 3
Another way to write it is
  2x + y = 9