`a) A = (7x + 3)/(9 - x^2) + (2x)/(x + 3) + (x + 1)/(x - 3)` 

`= (-(7x + 3))/(x^2 - 9) + (2x . (x - 3))/((x - 3) . (x + 3)) + ((x + 1) . (x + 3))/((x - 3) . (x + 3))` 

`= (-7x - 3)/((x - 3) . (x + 3)) + (2x^2 - 6x)/((x - 3) . (x + 3)) + (x^2 + 4x + 3)/((x - 3) . (x + 3))` 

`= (-7x - 3 + 2x^2 - 6x + x^2 + 4x + 3)/((x - 3) . (x + 3))` 

`= (3x^2 - 9x)/((x - 3) . (x + 3))` 

`= (3x . (x - 3))/((x - 3) . (x + 3))` 

`= (3x)/(x + 3)` 

`------`

`b)` Để `A ∈ ZZ` thì `3x \vdots x + 3` 

`=> 3 . (x + 3) - 9 \vdots x + 3` 

`Vì: 3 . (x + 3) \vdots x + 3` 

`=> 9 \vdots x + 3` 

`=> x + 3 ∈ Ư(9) = {1; -1; 3; -3; 9; -9}` 

`=> x ∈ {-2; -4; 0; -6; 6; -12}` 

`Vậy: x ∈ {-2; 4; 0; -6; 6; -12}` 

`------`

`c) P = A . B` 

`= (3x)/(x + 3) . (x^2 + 7)/(3x)`

`= (x^2 + 7)/(x + 3)` 

`P = 2 => (x^2 + 7)/(x + 3) = 2` 

`=> x^2 + 7 = 2 . (x + 3)` 

`=> x^2 + 7 = 2x + 6` 

`=> x^2 - 2x + 7 - 6 = 0` 

`=> x^2 - 2x + 1 = 0` 

`=> (x - 1)^2 = 0` 

`=> x - 1 = 0` 

`=> x = 1` 

`Vậy: x = 1`