(-b/2a) = Vertex?

Hi!

You are correct with your formula, we get -b/2a = 5/2

But we must take it one step further to find not the maximum value of x but the maximum value assumed by the function, meaning f(5/2).

f(5/2) = -(5/2)² + 5 * 5/2 - 4 = 9/4

Hope this helps!