`(1-3x)^{2021} = (3x-1)^{2019}`
`(3x-1)^{2019} - (1-3x)^{2021} = 0`
`(3x-1)^{2019} + (3x-1)^{2021} = 0`
`(3x-1)^{2019} [1 + (3x-1)^2] = 0`
⇒ \(\left[ \begin{array}{l}(3x-1)^{2019}=0\\1+(3x-1)^{2}=0\end{array} \right.\)
⇒ \(\left[ \begin{array}{l}3x-1=0\\(3x-1)^2 = -1 (loại)\end{array} \right.\)
⇒ `3x=1`
⇒ `x=1/3`
Vậy `x=1/3`