ACT Practice Question: Quadratic equations and graphs

Try this ACT math practice question on quadratic equations and the graphs of the corresponding parabola:

If the parabola $y=-x^2 + bx – 8$ has its vertex on the $x-$axis, then $b$ must be:

1. $\quad \textrm{a positive integer}$
2. $\quad \textrm{a positive or a negative rational number}$
3. $\quad \textrm{a positive rational number}$
4. $\quad \textrm{a positive or a negative irrational number}$
5. $\quad \textrm{a negative irrational number}$