In the graph f(x)=(x^2-x-6)/(x^2-2x), the interval (0, 2) is a up facing parabola.

In the graph f(x)=(x^2-x)/(x^2-4), the interval (-2,2), is a down facing parabola.

Without a calculator, how would I know whether the parabola should face up or down? I don't want to test decimal points because I can't use a calculator.