Equivalent expressions SAT question

The correct answer is $9$. The given expression can be rewritten as $(5x^3-3)+(-1)(-4x^3+8)$. By applying the distributive property, this expression can be rewritten as $5x^3-3+4x^3+(-8)$, which is equivalent to $(5x^3+4x^3)+(-3+(-8))$. Adding like terms in this expression yields $9x^3-11$. Since it's given that $(5x^3-3)-(-4x^3+8)$ is equivalent to $b{x}^3-11$, it follows that $9x^3-11$ is equivalent to $b{x}^3-11$. Therefore, the coefficients of $x^3$ in these two expressions must be equivalent, and the value of $b$ must be $9$.