We call graphs of functions with a never-changing slope, lines. Graphs with changing
slopes, we call curves. In calculus, we study derivatives. The derivative of f(x) is a function, denoted f'(x), that describes the changing slope of the curve f at any given x value. For instance, if f'(4) = 1 then when x = 4, the steepness of the curve f matches that of a line whose slope equals 1. (i.e., the curve is as steep when 4x = as the line y = x is all the time). Suppose f'(x) = 2x³ - 9x² + 7x + 6. Find all the x values for which the curve f has no steepness (that is, the curve's slope matches that of a horizontal line with a slope of zero.)