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justaguide eNotes educator| Certified Educator

The expression (3a - b)^6 has to be expanded.

Use the binomial theorem: `(a + b)^n = sum_(k = 0)^n C(n, k) a^(n - k)*b^k`

This gives: (3a - b)^6 = b^6 - 18ab^5 + 135a^2b^4 - 540a^3b^3 + 1215*a^4*b^2 - 1458*a^5*b + 729*a^6

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