Substituting this simplified numerator back into the limit, factoring and canceling common factors, we have 3t+ 3 3a + 3 f'(a) = lim t+6 t-a = lim a + 6 t-a - 15t 15a ta (t+6)(a + 6)(t - a) = lim x ta (t+6)(a + 6) Substituting this simplified numerator back into the limit, factoring and canceling common factors, we have 3t+ 3 3a + 3 f'(a) = lim t+6 t-a = lim a + 6 t-a - 15t 15a ta (t+6)(a + 6)(t - a) = lim x ta (t+6)(a + 6)