SAT Question 38a43902: Systems of Linear Equations answer and explanation with Desmos solution

It's given in the first equation of the system that $y=-2x$. Substituting $-2x$ for $y$ in the second equation of the system yields $3x+\left(-2x\right)=40$. Combining like terms on the left-hand side of this equation yields $x=40$. Therefore, the value of $x$ is $40$.