1) Ta có: \(76-\left(2x-1\right)^2=51\)
\(\Leftrightarrow\left(2x-1\right)^2=25\)
\(\Leftrightarrow\left[{}\begin{matrix}2x-1=5\\2x-1=-5\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}2x=6\\2x=-4\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=3\\x=-2\end{matrix}\right.\)
2) Ta có: \(3x\cdot3-1=80\)
\(\Leftrightarrow9x=81\)
hay x=9