Question 461113
Assuming you started with
3a +b/7 + b = 3a/7
Multiply both sides by 7 to get rid of fractions.
21a +b +7b = 3a
Subtract 3a from both sides
21a +b +7b -3a = 0
Collect terms
18a +8b =0
Divid by 2
9a +4b = 0