Fit with implicit nonlinear function - Matlab

[Ras9]

Hi guys! I am trying to fit a function whose x data depends nonlinearly on the parameter of the fit and I am having hard time doing that!
I will explain better: from my experiment I was able to measure my ydata e my x0 array and I know that my xdata are:
x=x0+a/(1+4x^2), with a being a parameter of the fit that I need to find.
After found a I can try to fit my data knowing my fitting function F(x,b) with b another parameter of the fit.
I managed to find a "brute" way to find the best a parameter for my data, by simply making it change and measuring the difference between my fitting function and my ydata. But know that I have to add a second parameter to the fit I really don't know what to do! Any help?
Thanks a lot

 

[kreil]

It is difficult to ascertain your problem based on your post. Can you please post a self-contained piece of code instead? Then it will be easier for other people to run your code and help you fix it or add to it.