Axis of symmetry is x = 6
One of the roots is -1.5
On the graph, -1.5 is to the left of the axis of symmetry by 7.5 units. If we were to go 7.5 units the opposite direction from the axis of symmetry (right), we would get the other root. Simply:
6 + 7.5 = 13.5
Therefore the other root would be 13.5.
Hope this helps! :)
