SAT Question 88e13c8c: Linear Functions answer and explanation with Desmos solution

Choice C is correct. It's given that $f\left(x\right)$ is the total cost, in dollars, to lease a car from this dealership with a monthly payment of $x$ dollars. Therefore, the total cost, in dollars, to lease the car when the monthly payment is $$400$ is represented by the value of $f\left(x\right)$ when $x=400$. Substituting $400$ for $x$ in the equation $f\left(x\right)=36x+1,000$ yields $f\left(400\right)=36\left(400\right)+1,000$, or $f\left(400\right)=15,400$. Thus, when the monthly payment is $$400$, the total cost to lease a car is $$15,400$.