Answer: The factorisation of 49b^2 - 81 is (7b - 9)(7b + 9). This is because 49 and 81 are the squares of 7 and 9 respectively, so these must be multiplied together. b^2 is b x b. Finally, it must be -9 and +9 to make -81, because when the signs are the same the result is a positive number.
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