ok i understand how to solve the problem, but i dont get how to get the restrictions.

i have to solve this problem and then put restrictions on it

4a^2-b^2-2a+b / (2a+b)^2-1

the answer is 2a+b / 2a+b+1

but on my answer sheet the restrictions also says b is not equal to 1-2a and -(2a+1)

my teacher also said it is 2a+b not equal to -1

i dont know how to find restrictions -____-