`a)`
`x^2 + 5x = 0`
`⇒ x(x + 5) = 0`
`⇒` \(\left[ \begin{array}{l}x = 0\\x = -5\end{array} \right.\)
`Vậy S = {0;-5}`
`b)`
`2x(x - 3) + (x - 3) = 0`
`⇒ (2x + 1)(x - 3) = 0`
`⇒` \(\left[ \begin{array}{l}2x + 1 = 0\\x - 3 = 0\end{array} \right.\)
`⇒` \(\left[ \begin{array}{l}x = -1/2\\x = 3\end{array} \right.\)
`Vậy S = {-1/2 ; 3}`
`c)`
`(x + 2)(x - 1) - x + 1 = 0`
`⇒ (x + 2)(x - 1) - (x - 1) = 0`
`⇒ (x + 2 - 1)(x - 1) = 0`
`⇒ (x + 1)(x - 1) = 0`
`⇒` \(\left[ \begin{array}{l}x = -1\\x = 1\end{array} \right.\)
`Vậy S = {-1 ; 1}`
`d)`
`x + 3 = (x + 3)^2`
`⇒ (x + 3)^2 - (x + 3) = 0`
`⇒ (x + 3)(x + 3 - 1) = 0`
`⇒ (x + 3)(x + 2) = 0`
`⇒` \(\left[ \begin{array}{l}x = -3\\x = -2\end{array} \right.\)
`Vậy S = {-3 ; -2}`