`4x(x - 2021) - x + 2021 = 0`
`⇔ 4x(x - 2021) - (x - 2021) = 0`
`⇔ (x - 2021)(4x - 1) = 0`
`⇔`\(\left[ \begin{array}{l}x - 2021 = 0\\4x - 1 = 0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=2021\\x=1/4\end{array} \right.\)
`text(Vậy x = { 2021 ; 1/4 })`