The formulas (5.71) only apply when the sample times are symmetric around 0. When the sample points t1.......tn are equally spaced, so ti+1 - ti = h for all i = 1........n - 1, then there is a simple trick to convert the least squares problem into a symmetric form.
(a) Show that the translated sample points s1 = ti - where

is the average, are symmetric around 0.
(b) Suppose q(s) is the least squares polynomial for the data points (si, yi). Prove that p(t) q(t - ) is the least squares polynomial for the original data (ti, yi).
(c) Apply this method to find the least squares polynomials of degrees 1 and 2 for the following data:

y-8-6- 3