a: \(\left(x-1\right)^{x+6}=\left(x-1\right)^{x+2}\)
\(\Leftrightarrow\left(x-1\right)^{x+2}\cdot\left[\left(x-1\right)^4-1\right]=0\)
\(\Leftrightarrow\left(x-1\right)^{x+2}\cdot\left(x-2\right)\cdot x=0\)
hay \(x\in\left\{0;1;2\right\}\)
b: \(\left(x+20\right)^{100}+\left|y+4\right|=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+20=0\\y+4=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-20\\y=-4\end{matrix}\right.\)