Use the parabola tool to graph the quadratic function f(x)=(x−4)(x+2). Graph the parabola by first plotting its vertex and then plotting a second point on the parabola.

If the eqn of the parabola is f(x) = (x-4)(x+2), we need to mult. this out to find the coordinates of the vertex.

Then f(x) = x^2 + 2x - 4x - 8, and f(x) = x^2 - 2x - 8.

One way in which you can find the vertex is to calculate x = -b / (2a).  Here, that comes to
     
        -(-2)
x = ---------- = 1.  f(1) = 1^2 - 2(1) - 8 = - 9.  Vertex is at (1, -9).
        2(1) 

Find another point on the curve:  Let x=0, for which y=-8.  Then (0, -8) is on the curve.  Plot the vertex and this point and draw a smooth parabola thru both.