How to Solve a Cubic Equation, Part 3: General Depression and a New Covariant

For pt.2 see ibid., vol. 26, no. 4, p. 90-100 (2006). Our ultimate goal here is twofold. We want to get insight into how cubic polynomials work, what are the invariants/covariants and their geometric interpretations. This article also describes how a transformation in parameter space relates to the transformation in coefficient space