Question

how is there -1 can anyone explain if u can't explain so pls don't write nonsense ​

Answer 1

In the last step a-b has been taken common out of the two terms.

So when you take out a-b from the second term, you should write 1 because if you multiply it again, you must get a-b only.

Answer 2

explanation

here we are taking common in last step

and when we take something common than 1 lefts inside there we took a-b common than - 1 left inside