function rcalc = kin2F(x,xdata);

% assign variable names to make it easier to type rate equation below

kf = x(1);
kr = x(2);
KA = x(3);
KB = x(4);

cA = xdata(:,1);
cB = xdata(:,2);

% compute the "calculated" rate, rcalc

rcalc = (kf * cA - kr * cB) ./ (1 + KA * cA + KB * cB);

% dot . needed before / since numerator and denominator are
% matrices and you don't want matrix division, just
% division of individual elements in the matrices
% dot . NOT needed before the * here since k, KA, KB are scalars