You can use fittype to specify the type of function you want to fit
x = linspace(0, 2, 100);
y = 2*x.^2 + 3*x + 4;
ft1 = fittype('a*x^2+b*x+c');
fit1 = fit(x',y',ft1);
ft2 = fittype('a*x^2+2.5*x+c'); % <--- fix the coefficient of x
fit2 = fit(x',y',ft2);
