If two adjacent angles have their exterior sides in perpendicular lines, then the two angles are also perpendicular.
Both exterior and interior angles sum up from 90 - 180 degrees. Therefore, if an exterior angle is perpendicular, then the interior angle must also be perpendicular in order for them to sum up to that amount of degrees (90 - 180).
The easiest method is to find the x-intercepts of the quadratic function by substituting y=0 in the function and solving it to find the possible values of x. The number of all possible values of x is the number of x-intercepts of the graph.
