Ta có: x + 21 = x + 3 + 18

Vì x + 3 chia hết cho x + 3 nên để x + 21 chia hết cho x + 3 thì 18 chia hết cho x + 3

Hay x + 3 ∈ Ư(18)

Suy ra: x + 3 ∈ {±1; ±2; ±3; ±6; ±9; ±18}

x ∈ {–2; –1 ; 0 ; 3 ; 6 ; 15 ; –4 ; –5 ; –6 ; –9 ; –12 ; –21}

Vậy x ∈ {–2; –1 ; 0 ; 3 ; 6 ; 15 ; –4 ; –5 ; –6 ; –9 ; –12 ; –21}.