`(x-1)(x+7) = (1-x)(3-2x)`

`<=> (x-1)(x+7) - (1-x)(3-2x) = 0`

`<=> x^2 + 6x - 7 - (2x^2 - 5x + 3) = 0`

`<=> x^2 + 6x - 7 - 2x^2 + 5x - 3 = 0`

`<=> -x^2 + 11x - 10 = 0`

`<=> -x^2 + 10x + x - 10 = 0`

`<=> -x(x - 10) + (x-10) = 0`

`<=> (x-10)(-x + 1) = 0`

`<=> [(x - 10 = 0),(-x + 1 = 0):}`

`<=> [(x = 10),(x = 1):}`

Vậy `S = {10;1}`

_________________

`(x+6)(5-x) = (x-5)(7x + 8)`

`<=> (x+6)(5-x) - (x-5)(7x + 8) = 0`

`<=> (x+6)(5-x) + (5-x)(7x+8) = 0`

`<=> (5-x)(x+6+7x+8) = 0`

`<=> (5-x)(8x+14) = 0`

`<=> [(5 - x = 0),(8x + 14 = 0):}`

`<=> [(x = 5),(8x = -14):}`

`<=> [(x = 5),(x = - 7/4):}`

Vậy `S = {5; - 7/4}`

`\ttcolor{cyan}{@NeverGiveUp}`