Let f(x)=2x3-12x-30x-4. a. Find all points on the graph of f(x) at which the tangent line is horizontal. b. Find all points on the graph of f(x) at which the tangent line has slope 240. a. The tangent line is horizontal at the point(s) (Type an ordered pair. Use a comma to separate answers as needed.) b. The slope of the tangent line is 240 at the point(s) (Type an ordered pair. Use a comma to separate answers as needed.)