if you notice from section a), by completing the square, you really ended up with the "vertex form of a parabola".
where's the vertex anyway? check the picture below.
now, the parabola has a leading term, namely x², with a positive coefficient, that means the parabola opens upwards, as in the picture.
so f(x), or namely "y", gets values from y = 4, 5, 6 , 7 .... +∞, since it goes up and up and up, and never down. Since it began above the x-axis, is never negative, since it never goes below 4 anyway.