midpoint is the direct center of a line so one side is equal to the other. (ex. if line ABC has a midpoint of B And AB = 6x+2, the AC = 6x+2+6x+2,) (or find the value of x if ABC has B as the midpoint and AB= 6x-3 and BC= 3x+6 then you use the equation 6x-3=3x+6 because they are the same length and solve for x)