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s³ + s² - 8m³ - 4m²
Swap the middle two terms:
s³ - 8m³ + s² - 4m²
Rewrite as
(s³ - 8m³) + (s² - 4m²)
The first parenthese contains a difference of two cubes and
the second parentheses contains a difference of two squares.
You have studied how to factor these.
(s - 2m)(s² + 2sm + 4m²) + (s - 2m)(s + 2m)
Next we factor out the common factor (s - 2m) using brackets:
(s - 2m)[(s² + 2sm + 4m²) + (s + 2m)]
Remove the parentheses inside the brackets
(s - 2m)[s² + 2sm + 4m² + s + 2m]
Change the brackets to parentheses:
(s - 2m)(s² + 2sm + 4m² + s + 2m)
Edwin