Lời giải:
ĐKXĐ: $9x^2+6x+1\geq 0$
$\Leftrightarrow (3x+1)^2\geq 0$
$\Leftrightarrow x\in\mathbb{R}$
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$\sqrt{9x^2+6x+1}=2-x$
\(\Rightarrow \left\{\begin{matrix} 2-x\geq 0\\ 9x^2+6x+1=(2-x)^2\end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} x\leq 2\\ 9x^2+6x+1=x^2-4x+4\end{matrix}\right.\)
\(\Leftrightarrow \left\{\begin{matrix} x\leq 2\\ 8x^2+10x-3=0\end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} x\leq 2\\ (4x-1)(2x+3)=0\end{matrix}\right.\Leftrightarrow x=\frac{1}{4}\) hoặc $x=\frac{-3}{2}$
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