When you open the brackets, you did not change the symbol from + into -.
He didn't change the symbols from + to -. Hence, it is wrong. So, this is due to lack of practice
Forgot to change the '-' and '+' signs when you opened the brackets.
He did not change the sign when opening the brackets.
You did not change the + symbol to a - after opening brackets..... Careless(I think)
At the third step , the signs of the numbers in the brackets should change as the positive numbers in the brackets are multiply by -3.As a result, the steps after that is wrong.
When opening the RHS bracket, the symbols were not changed accordingly.
I did not change the + symbol to a - after opening brackets... (Carelessness)
After opening the brackets , the signs should change .
It should be - at the back instead as when you open brackets you have to change the symbol accordingly
He did not change the + symbol to -.
You bo change + to -
for the 12a (4th line)he/she should change the operation from + to -as - and + is -
Open bracket never change sign ._.
The person did not change the sign.