( 5 x 3 - 3 ) - ( - 4 x 3 + 8 )

The correct answer is $9$. The given expression can be rewritten as $\left(5x^3-3\right)+\left(-1\right)\left(-4x^3+8\right)$. By applying the distributive property, this expression can be rewritten as $5x^3-3+4x^3+\left(-8\right)$, which is equivalent to $\left(5x^3+4x^3\right)+\left(-3+\left(-8\right)\right)$. Adding like terms in this expression yields $9x^3-11$. Since it's given that $\left(5x^3-3\right)-\left(-4x^3+8\right)$ 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 $9$.