a) Vì 84 ⋮ x; 180 ⋮ x nên x ∈ ƯC(84, 180)

Có: 84 = 2².3.7; 180 = 3².2².5; ƯCLN(84, 180) = 2².3 = 12

⇒ x ∈ ƯC(84, 180) = Ư(12) = {1; 2; 3; 4; 6; 12}

Vì x > 6 nên A = {12}.

b) Vì x $\vdots$ 12; x $\vdots$ 15; x $\vdots$ 18 nên x ∈ BC(12, 15, 18).

12 =  2².3; 15 = 3.5; 18 = 3².2; BCNN(12, 15, 18) =  2².5.3²  = 180

⇒ x ∈ BC(12, 15, 18) = B(180) = {0; 180; 360; 540; 720;...} 

Vì 0 < x < 300 nên B = {0; 180}