Estimate Margules model parameters using data regression

I am trying to construct a matrix of modeling functions and the right hand side by concatenating the two equations: Ln(gamma1) = [A12 + 2*(A21-A12)*x1]*x2^2 and Ln(gamma2) = [A21 + 2*(A12-A21)*x2]*x1^2. Then do regression to determine the fitting parameters and plot it up. In line 12 (x=k*y), I keep getting [NaN,NaN]' as my answer. Any suggestions to rectify this issue? I am attaching my solution and the original problem. Thanks!

My Code:

Original Problem: