Question

The function f is defined by f(x) = (x + 3)(x + 1).

The graph of f in the xy-plane is a parabola. Which
of the following intervals contains the x-coordinate
of the vertex of the graph of f?

Answer 1

x = -3 or x = -1

Explanation:

f(x) = (x + 3)(x + 1)

⇒ x = -3 or x = -1

Answer 2

Below in bold.

Explanation:

The zeroes ( = x-intercepts) are x = -3 and x = -1.

So the x -coordinate of the vertex is between x = -3 and x = -1.

The actual x-coordinate is halfway between these value: at x = -2.