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(6a - 7b)³

Using the formula,

[∵ (x - y)³ = x³ - y³ - 3x²y + 3xy²]

So,

⇒ (6a - 7b)³ = (6a)³ - (7b)³ - 3 $\times$ (6a)² $\times$ (7b) + 3 $\times$(6a) $\times$ (7b)²

= 216a³ - 343b³ - 756a²b + 882ab²

Hence, (6a - 7b)³ = 216a³ - 343b³ - 756a²b + 882ab².