Parentheses can be added or deleted between operations at the same level, that is, between addition and subtraction. If there is a+sign in front of parentheses, parentheses will have the same sign, but if there is a-sign in front of parentheses, parentheses will have a sign change.

Because there is no sign in front of the first bracket, it is actually a+sign, and the second bracket is a-sign, so in the second step, the symbol in the first bracket remains unchanged, and the-sign in the second bracket becomes a+sign.

Then, between operations at the same level, you can move with the symbol, which means the third step. The symbol in front of each number will not change.

The fourth step is to add parentheses, which are preceded by a+sign, so the symbol remains unchanged.

Finally, the common factor is extracted and calculated.