SAT Question 7e3f8363: Linear Functions answer and explanation with Desmos solution

Choice C is correct. In the xy-plane, an equation of the graph of a linear function can be written in the form $f\left(x\right)=mx+b$, where $m$ represents the slope and $\left(0,b\right)$ represents the y-intercept of the graph of $y=f\left(x\right)$. It's given that the graph of the linear function $f$, where $y=f\left(x\right)$, in the xy-plane contains the point $\left(0,3\right)$. Thus, $b=3$. The slope of the graph of a line containing any two points $\left(x_1,y_1\right)$ and $\left(x_2,y_2\right)$ can be found using the slope formula, $m=\frac{y_2-y_1}{x_2-x_1}$. Since it's given that the graph of the linear function $f$ contains the points $\left(0,3\right)$ and $\left(7,31\right)$, it follows that the slope of the graph of the line containing these points is $m=\frac{31-3}{7-0}$, or $m=4$. Substituting $4$ for $m$ and $3$ for $b$ in $f\left(x\right)=mx+b$ yields $f\left(x\right)=4x+3$.