(A + B)^{3} = A^{3 }+ 3A^{2}B^{ }+^{ }3AB^{2} + B^{3}

So, [(9+h)^{3} - 729]/h = [729 + 243h + 27h^{2} + h^{3 } - 729]/h

= [h^{3} + 27h^{2} + 243h]/h

= h^{2} + 27h + 243

lim [ ((9+h)^{3} - 729)/h ] (as h→0) = lim [h^{2 }+ 27h + 243] (as h→0)

= 0 + 0 + 243 = 243