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Isn’t there a mistake here?

Error 1: Addition must be performed before subtraction.

This is not always the case. If addition and subtraction are adjacent operations, the operations must be performed from left to right. For example, although addition and subtraction are both present in

5 – (-2) + 7 should read 5 – (-2) + 9

subtraction must be performed first. Performing the operation, we have

5 – (-2) + 9 = 7 + 9 = 16

There is no error. That is correct. You can try it using a spreadsheet.