SAT Question 939fc21b: Systems of Linear Equations answer and explanation with Desmos solution

Choice B is correct. The second equation in the given system is $r=3$. Substituting $3$ for $r$ in the first equation in the given system yields $s+7\left(3\right)=27$, or $s+21=27$. Subtracting $21$ from both sides of this equation yields $s=6$. Therefore, the solution $\left(r,s\right)$ to the given system of equations is $\left(3,6\right)$.