f(x) = 4x - x² - 2x
Vertex form of a parabola: y = a(x - h)² + k where (h, k) is the vertex of this parabola
y = a.(x - h)² + k
y = -1.(x - h)² + k
Let's factor:
f(x) = -x² - 2x + 4
f(x) = -(x + 1)² + 1 + 4
f(x) = -(x + 1)² + 5
So we don't even need to find the vertex, we can just factor.