From the sum of 3a−b+9 and −b−9 , subtract 3a−b−9

Solution

Given: expressions 3a−b+9 , −b−9 , 3a−b−9
We need to subtract 3a−b−9 from the sum of 3a−b+9 and −b−9
The sum of the first two terms, −b−9 and 3a−b+9 will be
3a−b+9+(−b−9)=3a−b+9−b−9=3a−2b
Now subtracting 3a−b+9 from 3a−2b , we get
3a−2b−(3a−b−9)
=3a−2b−3a+b=9=−b+9