Câu trả lời:

\(\left(2x+1\right)^4=81=3^4=\left(-3\right)^4\)

\(\Rightarrow\hept{\begin{cases}2x+1=3\\2x+1=-3\end{cases}\Rightarrow\hept{\begin{cases}2x=2\\2x=-4\end{cases}\Rightarrow}\hept{\begin{cases}x=1\\x=-2\end{cases}}}\)