Parametric proportional hazards model based on the Gompertz distribution

Question

[Yong]

Hi all, I'm reading a literature in which they estimated a phenotypic age through parametric proportional hazards model analysis with Gompertz distribution. The following are the statistical details:

But I still can not understand how they get the PhenotypicAge, for example, how to get the coefficient like b1 (which is not included in the step1). Is there anyone could give a more detailed derivation of the formulas, or another way to estimate the PhenotypicAge (Assuming that I have a series of phenotypic features).

Thank you !!!