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.
(a^{2} -b^{2} )=(a+b)*(a-b)
in this problem we have
49b^{2} - 81 =7^{2} b^{2} -9^{2} =(7b+9)*(7b-9)
so the answer would be (7b-9)(7b+9)
hope this helps you have a great day
