The x intercepts on the graph are the points where the curve cut across the y axis. At this points, y = 0
Given that the graph has exactly two x-intercepts, one at x=-6 and the other at x=3, it means that the factors are
(x + 6)(x - 3) = 0
We would multiply each term in the first bracket by each term in the second bracket. We have
x^2 - 3x + 6x - 18
x^2 + 3x - 18 = 0
The equation is
x^2 + 3x - 18 = 0
