a,
`3x \vdots x-1`
`<=>3x-3+3 \vdots x-1`
`<=>3(x-1)+3 \vdots x-1`
`<=>3 \vdots x-1` (vì `3(x-1) \vdots x-1`)
`=>x-1 \in Ư(3)`
`=>x-1 \in {-3;-1;1;3}`
`=>x \in {-2;0;2;4}`
Vậy `x \in {-2;0;2;4}` thì `3x \vdots x-1`
b,
`2x+7 \vdots x+2`
`<=>2x+4+3 \vdots x+2`
`<=>2(x+2)+3 \vdots x+2`
`<=> 3 \vdots x+2` (vì `2(x+2) \vdots x+2`)
`=>x+2 \in Ư(3)`
`=>x+2 \in {-3;-1;1;3}`
`=>x \in {-5;-3;-1;1}`
Vậy `x \in {-5;-3;-1;1}` thì `2x+7 \vdots x+2`
