well, let's first off, expand the expression, 3x(x-4) => 3x²-12x, which we can also write as
f(x) = 3x² - 12x + 0
now, this is a quadratic equation with a leading term with a positive coefficient, namely its graph is a parabola, or a Cup-Shaped figure, so the graph comes from the top, goes down down down, reaches a U-turn, then goes back up up up.
the U-turn, or vertex of the quadratic, is the lowest point in the "cup".
