Yours Truly answers:
We replace each  x  in the function with  t - 1  to have:

Note that the red parts are derived by using the formula  (a – b)² = a² – 2ab + b²  by letting  a = t   and  b = 1. Namely,  (t – 1)² = t² – 2·t·1 + 1² = t² – 2t + 1.