how to find unknown parameteres form data file

Hi! I have data file for y and x which gives nonlinear plot and I have the equation for the curve with 5unknown parameters in the equation. How can i find those 5 unknown parameters(a,b,c,d,z)?

y=(r+1i*w)=sqrt(1+(a/(1-(b/x).^2))+(c/((1-(d/x).^2)+((1i.*z).*(d./x)))));