(3/14+5/21x):3/7= 3 1/4
3/14:3/7+5/21х:3/7=3 1/4 1/2+5/9х=3 1/4 5/9х=3 1/4-1/2 5/9х=3,25-0,5 5/9х=2,75 х=4(целых)19/20