Explanation:
Use grouping.
f(x) = x⁴ + 2x³ − 9x² + 18x
f(x) = x³ (x + 2) − 9x (x + 2)
f(x) = (x³ − 9x) (x + 2)
f(x) = x (x² − 9) (x + 2)
f(x) = x (x − 3) (x + 3) (x + 2)
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