\(\left(x-3\right)^{2020}-\left(x-3\right)^{2021}=0\\ \left(x-3\right)^{2020}-\left(x-3\right)^{2020}.\left(x-3\right)=0\\ \left(x-3\right)^{2020}\left(1-x+3\right)=0\\ \left(x-3\right)^{2020}\left(4-x\right)=0\\ \left[{}\begin{matrix}\left(x-3\right)^{2020}=0\\4-x=0\end{matrix}\right.=>\left[{}\begin{matrix}x=3\\x=4\end{matrix}\right.\)