Factor completely:
Split the four terms into two useful pairs. Grouping works only if each pair has a common factor and the leftover brackets match. Pairing the first two and the last two:
Factor each pair. From the first pair take out ; from the second take out (the minus is deliberate, so the bracket comes out the same way round):
Had we taken out we would have got — correct, but it hides the match.
Confirm the brackets agree. Both pairs produced :
This agreement is the signal that the grouping was chosen correctly.
Pull the common binomial out.
Expand back and test numerically. , which is the original after reordering ✓. At , : the original is , and ✓. Neither factor breaks down further, so the factoring is complete.
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