SAT Question a91a2b75: Linear Functions answer and explanation with Desmos solution

The y-intercept of the graph of $y=f(x)$ in the xy-plane is the point where the graph of $y=f(x)$ crosses the y-axis, which occurs at $x=0$. It’s given that the function $f$ is defined by $f(x)=-9x+9$. Substituting $0$ for $x$ and $y$ for $f(x)$ in this equation yields $y=-9(0)+9$, or $y=9$. It follows that $y=9$ when $x=0$ and that the y-intercept of the graph of $y=f(x)$ in the xy-plane is $(0,9)$. Therefore, the y-coordinate of the y-intercept of the graph of $y=f(x)$ in the xy-plane is $9$.