3 pts
Identify the equation for the axis of symmetry of the parabola given by the equation below where $a$ is unknown.
\[
y=a(x-3)(x+5)
\]
$x=a$
$x=-1$
$x=2$
$x=-8$

Step 1 ：Identify the equation for the axis of symmetry of the parabola given by the equation below where $a$ is unknown. The equation is $y=a(x-3)(x+5)$.

Step 2 ：The axis of symmetry of a parabola given by the equation $y=a(x-h)^2+k$ is $x=h$. In this case, the equation is in the form $y=a(x-p)(x-q)$, which can be rewritten as $y=a(x^2-(p+q)x+pq)$.

Step 3 ：Comparing this with the standard form, we can see that $h$ is the average of $p$ and $q$. In this case, $p=3$ and $q=-5$, so the axis of symmetry is $x=\frac{p+q}{2}$.

Step 4 ：Substitute $p=3$ and $q=-5$ into the equation, we get $h=-1.0$.

Step 5 ：Final Answer: The equation for the axis of symmetry of the parabola is $x=\boxed{-1}$.