Đáp án:
$\begin{array}{l}
a)2x + 3 \ge x\\
\Leftrightarrow 2x - x \ge - 3\\
\Leftrightarrow x \ge - 3\\
\text{Vậy}\,x \ge - 3\\
b)y = \dfrac{{ - 3{x^2} + 5x + 1}}{{{x^2} - 16}}\\
Dkxd:{x^2} - 16 \ne 0\\
\Rightarrow {x^2} \ne 16\\
\Rightarrow \left\{ \begin{array}{l}
x \ne 4\\
x \ne - 4
\end{array} \right.\\
\text{Vậy}\,TXD:D = R\backslash \left\{ { - 4;4} \right\}
\end{array}$