Question 923964
Find slope of AB.  Find midpoint of AB.  Use point-slope form of equation for a line to derive the equation for the perpendicular bisector line.


You noticed that the slope of AB is 0, zero, and therefore you are finding an undefined slope for any line perpendicular to line AB  (and to segment AB).  This is then almost finished.  "Slope-Intercept" form for the equation you are looking for would not really be important.  You want the vertical line which contains the midpoint of segment AB.


Now after I see your newer personal note, you have identified the answer but do not recognize what you found.  NO SLOPE!  The slope is UNDEFINED.  The line is vertical.  You need to recognize that you found your answer for the equation.