SAT Question ea8bd66f: Systems of Linear Equations answer and explanation with Desmos solution

Choice D is correct. The first equation in the given system is $y=4x$. Substituting $4x$ for $y$ in the second equation yields $5x-3(4x)=-14$, which is equivalent to $5x-12x=-14$, or $-7x=-14$. Dividing both sides of this equation by $-7$ yields $x=2$. Substituting $2$ for $x$ in the equation $y=4x$ yields $y=4(2)$, or $y=8$. Substituting $2$ for $x$ and $8$ for $y$ in the expression $x+y$ yields $2+8$, or $10$.