SAT Question 73b3b7d8: Systems of Linear Equations answer and explanation with Desmos solution

Adding the first equation to the second equation in the given system yields $5y-5y=10x+5x+11-21$, or $0=15x-10$. Adding $10$ to both sides of this equation yields $10=15x$. Multiplying both sides of this equation by $2$ yields $20=30x$. Therefore, the value of $30x$ is $20$.