3/7х + 2/3х - 4/21х =
приводим дроби к общему знаменателю 21:
= 9/21х + 14/21х - 4/21х = 19/21х
если х = 3/19
19/21 * 3/19 = 3/21 = 1/7
если х = 7/38
19/21 * 7/38 = (1*1)/(3*2) = 1/6
