SAT Question 1362ccde: Systems of Linear Equations answer and explanation with Desmos solution

The first equation in the given system of equations defines $y$ as $4x+1$. Substituting $4x+1$ for $y$ in the second equation in the given system of equations yields $4\left(4x+1\right)=15x-8$. Applying the distributive property on the left-hand side of this equation yields $16x+4=15x-8$. Subtracting $15x$ from each side of this equation yields $x+4=-8$. Subtracting $4$ from each side of this equation yields $x=-12$. Substituting $-12$ for $x$ in the first equation of the given system of equations yields $y=4\left(-12\right)+1$, or $y=-47$. Substituting $-12$ for $x$ and $-47$ for $y$ into the expression $x-y$ yields $-12-\left(-47\right)$, or $35$.