(A + B)3 = A3 + 3A2B + 3AB2 + B3
So, [(9+h)3 - 729]/h = [729 + 243h + 27h2 + h3 - 729]/h
= [h3 + 27h2 + 243h]/h
= h2 + 27h + 243
lim [ ((9+h)3 - 729)/h ] (as h→0) = lim [h2 + 27h + 243] (as h→0)
= 0 + 0 + 243 = 243