Real Hypersurfaces In Complex Manifolds

Question

I have a problem:

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I don't understand (2.12) and (2.13) :( How to prove that $$PF=\sum_{\min(k,l) \le 1}F_{kl}+G_{11}\left \langle z,z \right \rangle +\left ( G_{10}+G_{01} \right )\left \langle z,z \right \rangle^2+G_{00}\left \langle z,z \right \rangle^3$$

(Source: Real Hypersurfaces In Complex Manifolds By S.S.Chern and J.K.Moser)

Any help will be appreciated! Thanks!