You'll have to include the working to show that t = 2 is where the maximum point is i.e second derivative test. Since the student isn't actually expected to draw the graph (to conclude it by inspection)

The student won't be able to tell without the graph of x³ - 9t² + 24t that the particle turns back at t = 2 and turns again at t = 4

But for 3x² - 18x + 24, a quadratic function , they would be able to tell that velocity is negative between t = 2 and t = 4 since the positive sign of the coefficient of x² suggests it opens upwards (smiling curve)