Patrick B. answered 10/18/19
Math and computer tutor/teacher
At x=-4, y = -3.53
At x = 8, y = 4.852
At x=16, y = 2.165
the derivative is x * exp(-x^2/128) (-x/64) + exp(-x^2/128)
= exp("-x^2/128) [ -x^2/64 + 1]
exponential is never zero, so -x^2/64 + 1 = 0 ---> x^2 = 64 ---> x = +or- 8 are the critical values