SAT Question bd45df49: Systems of Linear Equations answer and explanation with Desmos solution

Choice D is correct. The first equation in the given system of equations defines $y$ as $3x+9$. Substituting $3x+9$ for $y$ in the second equation in the given system of equations yields $3(3x+9)=8x-6$. Applying the distributive property on the left-hand side of this equation yields $9x+27=8x-6$. Subtracting $8x$ from both sides of this equation yields $x+27=-6$. Subtracting $27$ from both sides of this equation yields $x=-33$. Substituting $-33$ for $x$ in the first equation of the given system of equations yields $y=3(-33)+9$, or $y=-90$. Substituting $-33$ for $x$ and $-90$ for $y$ into the expression $x-y$ yields $-33-(-90)$, or $57$.