f(x) = If[0 ≤ x ≤ 12.5, (0.14 * x^(3)) - (3.15 * x^(2)) + (18.48 * x) + 18]
f'(x) = If[0 ≤ x ≤ 12.5, ((0.42 * (x - 4)) * (x - 11))]