Answer: b=15
Explanation:(9+11)+15=9+(11+b)
Add 9 and 11 to get 20.
20+15=9+11+b
Add 20 and 15 to get 35.
35=9+11+b
Add 9 and 11 to get 20.
35=20+b
Swap sides so that all variable terms are on the left hand side.
20+b=35
Subtract 20 from both sides.
b=35−20
Subtract 20 from 35 to get 15.
b=15