Quadratic Functions Exercise 3 - Finding a Maximum of a Quadratic Function

The equation giving the vertical position y as a function of the horizontal position x for a projectile launched at a 45 degree angle with initial velocity V_o is y = (-32/V_o²)x² + x.  If the initial velocity is V_o=20 ft/sec, what is the maximum height that this projectile achieves? The function and it's graph are given below. The justification has been given - no further justification is needed.
 

STATEMENT	JUSTIFICATION
	Given   Substitute in the given value of Vo.   The vertex of quadratic functions of the form y=ax2 + bx + c will be at x=-b/(2a).   The maximum y-value occurs at the vertex x-value.