math
posted by hannah .
Stephanie is working on a graph that represents the path of a roller coaster. The point (0, 840) represents the point at which the roller coaster first begins to drop at a constant rate. The point (15, 240) represents a point along the ride at which the roller coaster is still dropping. If the xintercept represents the point at which the ride stops dropping, what is the xintercept?

since y decreased by 600 when x increased by 15,
y = 840  40x
now solve for x when y=0