@KH2020H0 We know (a-b)³= a³-3a²b+3ab²-b³ Then a= 3x, b = 1, -> (3x - 1)³ And bottom is (3x-1)² So result is 3x-1
@KH2020H0 Since when evaluate all of the options at x=0 you get different results, if you evaluate the expression at the the top at x=0 you get -1, so the answer is B.
@KH2020H0 A favorite trick: The answer must be independent of the value of x. So try some values: If x = 0, the expression = - 1. If x = 1, the expression = 2. The only choice that gives these results is choice (b): 3x - 1