I just have of about 14 points as a data

I need to fiting them in a polynomial called Redlich- kister

in the form

y=x(1-x)∑(a0+a1(2x-1)+a2(2x-1)^2+a3(2x-1)^3)

I tried to solve it in FORTRAN with least square but i dint got a good answer for that

I got a program for solving the normal one as

a0+a1(2x-1)+a2(2x-1)^2+a3(2x-1)^3)

please help and if you have a code will much better

thankssssssss