Question:
Find all points at which the complex valued function $f$ define by $f(z)=(2+i)z^3-iz^2+4z-(1+7i)$ has a derivative.
I know that $z^3$,$z^2$, and $z$ are differentiable everywhere in the domain of $f$, but how can I write my answer formally? Please can somebody help?
Note:I want to solve the problem without using Cauchy-Riemann equations.