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:

- $\quad \textrm{a positive integer}$
- $\quad \textrm{a positive or a negative rational number}$
- $\quad \textrm{a positive rational number}$
- $\quad \textrm{a positive or a negative irrational number}$
- $\quad \textrm{a negative irrational number}$

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