SAT Mathematics Practice Test 3

The parabola with equation $\mathit{y}=\mathit{a}\left(\mathit{x}-2\right)\left(\mathit{x}+4\right)$ crosses the x-axis at the points $0\left(-4,0\right)$ and $\left(2,0\right)$.
The x-coordinate of the vertex of the parabola is halfway between the x-coordinates of $\left(-4,0\right)$ and (2, 0).
Thus, the x-coordinate of the vertex is $\frac{-4+2}{2}=-1$.
This is the value of c. To find the y-coordinate of the vertex, substitute $-1$ for $\mathit{x}$ in
Therefore, the value of d is $-9\mathit{a}.$
Choice B is incorrect because the value of the constant term in the equation is not the y-coordinate of the vertex, unless there were no linear terms in the quadratic.
Choice C is incorrect and may be the result of a sign error in finding the x-coordinate of the vertex.
Choice D is incorrect because the negative of the coefficient of the linear term in the quadratic is not the y-coordinate of the vertex