hi i'm struggling with this:

when a parabola crosses the x-axis at the points A and B, the axis of symmetry is the perpendicular bisector of the line joining the points A and B. since A is the point (-5, 0) and B is the point (2/3, 0) the equation of the line of symmetry is x = -13/6

i know that the mid point is -13/6 but why is that the equation? i thought you had to go through the whole y - y1 = m(x - x1) business. when the gradient of a horizontal line is 0 how do you come to the conclusion that the equation is x = -13/6.

thanks for any help