The inverse function, Xi=f(Y, Xj)

Hi, I am working on a function with 6 variables (A B C D E F) and ONE output Gmax. I want now to study my function differently. I am looking for a way to find E=f(gmax, a, b, c, d, F)with gmax, a, b, c, d are fixed. I want to get the points (E,F) and their trends knowing that (E,F) is linear or polynomial of second ordre. My function is attached and the variation screens of the 6 variables are put in comment in the code.

Thank you a lot.