Mark M. answered 10/02/21
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
The tangent line is horizontal only when f'(x) = 0.
So, ex^3 - 4x^2 - 8x + 10(3x2-8x-8) = 0
e to a power is always positive, so 3x2-8x-8 = 0
Use the quadratic formula to solve for x.