6. a. Use an x− interval of 0.0001 to determine the instantaneous range of change of the function f(x)= x4+ 8x3 +19x2+12xatx = −2. Give answer to the nearest tenth. [A3] b. Determine whether the function f(x)= x4+ 8x3 + 19x2+12x at x = − 2 has a maximum or minimum at the point where x = − 2. Justify your answer. [A5] [C2]