SAT Question 686b7cad: Systems of Linear Equations answer and explanation with Desmos solution

Choice A is correct. It's given that a radio show stated that $3$ times as many people voted in favor of the proposal as people who voted against it. Let $x$ represent the number of people who voted against the proposal. It follows that $3x$ is the number of people who voted in favor of the proposal and $3x-x$, or $2x$, is how many more people voted in favor of the proposal than voted against it. It's also given that a social media post reported that $15,000$ more people voted in favor of the proposal than voted against it. Thus, $2x=15,000$. Since $2x=15,000$, the value of $x$ must be half of $15,000$, or $7,500$. Therefore, $7,500$ people voted against the proposal.