Giúp e với cảm ơn ạ !

`a)` 
`2 . ( 3x - 18 ) - 3 . ( 4x - 15 ) = 4 . ( 5x - 4 ) + 21` 
`<=> 6x - 36 - 12x + 45 = 20x - 16 + 21` 
`<=> -6x - 20x = 5 + 36 - 45` 
`<=> -26x = -4` 
`<=> x = 2/13` 
Vậy `S = { 2/13 }`
`b)`
`5 . ( 2x - 3 ) + 4x . ( x - 2 ) + 2x . ( 3 - 2x ) = 0` 
`<=> 10x - 15 + 4x^2 - 8x + 6x - 4x^2 = 0`
`<=> 8x - 15 = 0` 
`<=> 8x = 15` 
`<=> x = 15/8`
Vậy `S = { 15/8 }`
`c)` 
`3x . ( 2 - x ) + 2x . ( x - 1 ) = 5x . ( x + 3 )` 
`=> 6x - 3x^2 + 2x^2 - 2x = 5x^2 + 15x`
`<=> x^2 - 6x^2 + 6x - 2x - 15x = 0` 
`<=> -6x^2 -11x = 0` 

`<=> x . ( -6x - 11 ) = 0`
`<=>` \(\left[ \begin{array}{l}x=0\\-6x-11=0\end{array} \right.\) 
`<=>` \(\left[ \begin{array}{l}x=0\\x=\dfrac{-11}{6}\end{array} \right.\) 
Vậy `S = { 0 ; -11/6 }`