`1)x^3-5x + a vdots x-3`   `(x ne 3)`

`x^3 - 3x^2 + 3x^2 - 9x + 4x - 12 +12 +x vdots x-3`

`=>x^2(x-3)+ 3x(x-3) + 4(x-3) +12+x vdots x-3`

`=>(x^2+3x+4)(x-3) +12+x vdots x-3`

`=>12+xvdots x-3`

Để `x^2 - 5x +a vdots x-3` thì 

`12+x =0`

`=>x=-12`

`2)2x^2 +x + a vdots x+3`

`=>2x^2 + 6x - 5x - 15 + 15+a vdots x+3`

`=>2x(x+3) - 5(x+3) +15+a vdots x+3`

`=>(2a-5)(x+3) +15 +a vdots x+3`

`=>15+a vdots x+3`

Để `2x^2 +x +a vdots x+3` thì

`15+a=0`

`a=-15`

`3)x^3 + 2x^2 + a vdots x+3`

`x^3 + 9x^2 - 7x^2 - 21x + 21x + 63 + a-63 vdots x+3`

`x^2(x+3) - 7x(x+3) + 21(x+3)+ a-63 vdots x+3`

`(x^2-7x+21)(x+3) +a-63 vdots x+3`

`=>a-63 vdots x+3`

Để `x^3 + 2x^2 + a vdots x+3` thì

`a-63=0`

`a=63`