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 $b = 7$.

Choice A is incorrect and may be the result of a conceptual error when finding the value of b. Choices C and D are incorrect and may result from making a sign error when simplifying.