\(\Leftrightarrow20x-6\left(x+1\right)=60+15\left(x-3\right)\)
=>20x-6x-6=60+15x-45
=>15x+15=14x-6
=>x=-21
`<=> (20x)/30 - (6x + 6)/30 = 60/30 + (30x - 90)/30`
`<=> (14x - 6)/30 = (30x - 30)/30`
`=> (7x - 3)/10 = (10x-10)/10`
`=> 7x - 3 = 10x - 10`
`=> 13 = 3x`
`=> x = 13/3`
\(\Leftrightarrow\dfrac{20x}{30}-\dfrac{6\left(x+1\right)}{30}=\dfrac{60}{30}+\dfrac{15\left(x-3\right)}{30}\)
\(\Leftrightarrow20x-6x-6-60-15x+45=0\)
<=>-9 -x = 0
<=> x = -9 - 0 = -9
=> x = - 9
<=> \(\dfrac{10.2x}{30}\) - \(\dfrac{6\left(x+1\right)}{30}\) = \(\dfrac{30.2}{30}\) + \(\dfrac{15\left(x-3\right)}{30}\)
<=> 20x - 6x - 6 = 60 +15x - 45
<=> 20x - 6x - 6 - 60 - 15 + 45 = 0
<=> - x - 21 = 0
<=> - x = 21
<=> x = -21
Vậy S = \(\left\{-21\right\}\)
