a) Vì 84 $\vdots$ x, 180 $\vdots$ x nên x $\in$ ƯC(84, 180).

84 = 2².3.7; 180 = 2².3².5; ƯCLN(84, 180) = 2².3 = 12.

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 $\in$ BC(12, 15, 18).

12 = 2².3; 15 = 3.5; 18 = 2.3²; BCNN(12, 15, 18) = 2².3².5 = 180.

BC(12, 15, 18) = B(180) = {0; 180; 360; 540;…}

Vì 0 < x < 300 nên B = {180}.