`a)`

`12/{x-1} = 4/3`

`=> 4(x-1) = 36`

`=>x-1 = 9`

`=> x= 10`

`{4-2y}/9 = 4/3`

`=> 3(4-2y) = 36`

`=> 4-2y = 12`

`=> 2y = -8`

`=> y = -4`

Vậy `(x;y) = (10;-4)`

`b)`

`xy + 3x  + 2y = 5`

`xy + 3x + 2y + 6 = 11`

`x(y+3) + 2(y+3) = 11`

`(x+2)(y+3) =11`

`TH1:`

`x+2 = 1 => x = -1`

`y+3 = 11 => y = 8`

`TH2:` 

`x + 2 = 11 => x = 9`

`y+3 = 1 => y = -2`

`TH3:`

`x+2 = -1 => x = -3`

`y + 3 = -11 => y = -14`

`TH4:`

`x + 2 = -11 => x = -13`

`y + 3 =-1 => y = -4`

Vậy `...`