Hi, i couldn't help but notice this is part of the modelling nature's nonlinearity assignment?
I was just wondering how far along you were and if you could help me because im struggling on it!
Also, in regards to your question, i think you need ONE equation in ONE variable, not TWO as you have with x1 and x2? i hope this helps, you probably have your answer already anyway,
If you could help, i would really appreciate it.