Explanation:
-2x³ + 14x² + 120x
= -2x(x² - 7x - 60)
= -2x(x - 12)(x + 5).
The other 2 dimensions are -2x and (x + 5).
When using a graphing calculator to maximise the volume of the prism, we get x = 7.378. However this value is not reasonable as it makes -2x and (x - 12) negative, which are the sides of the prism.