6/7 : х = 18/21 : 1/2

x = 1/2

Reason: Treat (6/7) : x = (18/21) : (1/2) as a proportion a:b = c:d, so ad = bc: (6/7) * (1/2) = x * (18/21) 3/7 = x * (6/7) 7 both sides: 3 = 6x → x = 1/2.