How to expand an expression modulo?

Question

Mathematics

The image shows the solution. The last step is not clear. Why does the sign change in each case in the expression? In the first case nothing was changed, in the second case the sign before the second parenthesis was changed, and in the third case the sign was changed before both parentheses. Clarify please.

Answer 1

Maybe because there were brackets-sticks, and there were brackets-brackets?