To simplify the expression, we distribute terms accordingly. It is done as follows:
(3a – 4b)(a + b)
3a² + 3ab -4ab -4b²
3a² - ab -4b²
Therefore, the correct answer is option A, 3a^2– ab – 4b^2.
Hope this answers the question. Have a nice day. Please feel free to ask more questions.