SAT Question 79784c23: Systems of Linear Equations answer and explanation with Desmos solution

Choice D is correct. It's given that the system has infinitely many solutions. A system of two linear equations has infinitely many solutions when the two linear equations are equivalent. When one equation is a multiple of another equation, the two equations are equivalent. Multiplying each side of the given equation by $2$ yields $2\left(y\right)=2\left(6x+3\right)$. Thus, $2\left(y\right)=2\left(6x+3\right)$ is equivalent to the given equation and could be the second equation in the system.