Determine the following limit$.$

$\underset{x}{lim}\frac{21x^{3}2x^2-3x}{28x^3{x}^{2}3x1}$

Select the correct choice below and, If necessary, fill in the answer box to complete your choice.

A$.\underset{x}{lim}\frac{21x^{3}2x^2-3x}{28x^3{x}^{2}3x1}=($Simplify your answer.$)$

$8.$ The limit as $x$ approaches $$ does not exist and is neither $$ nor $-.$