SAT Question 271f7e3f: Linear Functions answer and explanation with Desmos solution

Choice B is correct. The value of $f(9)-f(1)$ can be calculated by finding the values of $f(9)$ and $f(1)$. The value of $f(9)$ can be found by substituting $9$ for $x$ in the given function: $f(9)=\frac{9+7}{4}$. This equation can be rewritten as $f(9)=\frac{16}{4}$, or $4$. Then, the value of $f(1)$ can be found by substituting $1$ for $x$ in the given function: $f(1)=\frac{1+7}{4}$. This equation can be rewritten as $f(1)=\frac{8}{4}$, or $2$. Therefore, $f(9)-f(1)=4-2$, which is equivalent to $2$.