SAT math practice question: Properties of parabolas

 
Try the following SAT math question (No Calculator) that tests your understanding of the graph of parabolas in the coordinate plane.

$$ y = (x-r)(x-s)$$
The equation above represents the graph of a parabola in the $xy-$plane, where $r$ and $s$ are constants, and $r \neq s$. Which of the following represents the $x-$coordinate of the vertex of the parabola?

1. $\quad \dfrac{r-s}{2}$
2. $\quad \dfrac{r+s}{2}$
3. $\quad r-s$
4. $\quad r+s$