Not sure I follow. The aim IS that the term on the right simplifies every time ... to avoid 'multiplying out' the subtrahend bracket.
[So I look at the subtrahend (7−2x) and I 'restore' the 7 by doing a '+2x', and then I do a '+2x' to the minuend. If the subtrahend was (7+2x) I'd be doing '−2x'].
