SAT Question 8a6de407: Linear Functions answer and explanation with Desmos solution

The slope-intercept form of an equation for a line is $y=mx+b$, where $m$ is the slope and $b$ is the $y$-coordinate of the $y$-intercept. Two ordered pairs, $(x_1,y_1)$ and $(x_2,y_2)$, can be used to compute the slope using the formula $m=\frac{y_2-y_1}{x_2-x_1}$. It's given that $f(0)=18$ and $f(1)=20$; therefore, the two ordered pairs for this line are $(0,18)$ and $(1,20)$. Substituting these values for $(x_1,y_1)$ and $(x_2,y_2)$ gives $\frac{20-18}{1-0}=\frac{2}{1}$, or $2$.