`(x+6)(680/x - 3) = 680` (ĐK: `x ne 0`)

`<=> x(680/x - 3) + 6(680/x - 3) = 680`

`<=> 680 - 3x + 4080/x - 18 - 680 = 0`

`<=> -3x + 4080/x - 18 = 0`

`<=> (-3x^2 + 4080 - 18x)/x = 0`

`<=> -3x^2 + 4080 - 18x = 0`

`<=> -3(x^2 + 18x - 4080) = 0`

`<=> x^2 + 18x - 4080 = 0`

`<=> x^2 + 40x - 34x - 4080 = 0`

`<=> x(x+40) - 34(x+40) = 0`

`<=> (x+40)(x-34) = 0`

`<=> [(x + 40 = 0),(x - 34 = 0):}`

`<=> [(x = -40),(x = 34):}`

Vậy `S = {-40;34}`

`\ttcolor{cyan}{#PUyen}`