Let (x) = x 2 + 2x 21. (a) Show that the lines y = 3x

Let ƒ(x) = x² + 2x − 21.
(a) Show that the lines y = 3x − 25, y = 6x − 25, and y = 9x − 25 intersect the graph of ƒ in zero, one, and two points, respectively.
(b) Sketch the graph of f and the three lines from (a).
(c) Describe the relationship between the graph of ƒ and the lines y = cx − 25 as c changes from 0 to ∞.