SAT Question 70e29454: Linear Equations In One Variable answer and explanation with Desmos solution

Choice B is correct. Distributing the $a$ on the left-hand side of the equation gives $3a-b-ax=-1-2x$. Rearranging the terms in each side of the equation yields $-ax+3a-b=-2x-1$. Since the equation has infinitely many solutions, it follows that the coefficients of $x$ and the free terms on both sides must be equal. That is, $-a=-2$, or $a=2$, and $3a-b=-1$. Substituting $2$ for $a$ in the equation $3a-b=-1$ gives $3(2)-b=-1$, so $b=7$.