b, ĐKXĐ: \(x\ge-\frac{3}{4}\)
\(pt\Leftrightarrow x+1+4x+3+2\sqrt{\left(x+1\right)\left(4x+3\right)}=3x+12\)
\(\Leftrightarrow\sqrt{4x^2+7x+3}=4-x\)
\(\Leftrightarrow\left\{{}\begin{matrix}4x^2+7x+3=\left(4-x\right)^2\\4-x\ge0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}3x^2+15x-13=0\\x\le4\end{matrix}\right.\Leftrightarrow x=\frac{-15+\sqrt{381}}{6}\left(tm\right)\)