Explanation:
there is something wrong with your problem description.
you really mean x² - 6x² + 16 ?
or is it 2x - 6x² + 16 ?
in any case this would be a squared equation, which does not have 2 turning points.
we would need an equation with x³.
for that we would create the 1st deviation, which would be then a squared equation with 2 solutions, which would be the 2 turning points of the initial equation.