3 3/8 разделить на (1/14x+5/7)=2.1

3 3/8: (1/14x + 5/7) = 2,1
1/14x + 5/7 = 3 3/8: 2 1/10

3 3/8: 2 1/10 = 27/8 : 21/10= 27/8*10/21=45/28 = 1 17/28
1/14x + 5/7 = 1 17/28
1/14x = 1 17/28 - 5/7
1/14x = 1 17/28 - 20/28
1/14x = 25/28
x = 25/28:1/14 = 25/28*14/1= 12,5