@Anonymous1234567890,
												Well , the numerical difference between our results is due to the fact that on the first hand you gave an approx. mean , it should be (51/49)+57 which is not exactly 58.04 . Base change is a term  we commonly use  in statistics , your variables were x= 51,54,57,60,63   I  said to change it to y= x - 57  , so the  new  variables  are  -6(=51-57) , -3(=54-57) ...etc.  Also  y= 57 - x  →     fy= fx - 57f   ( for all  f ,x,y) summing  over all the values we get ∑ fy 
=  ∑fx  - 57 ∑f , dividing by  ∑f  we  get  mean(y) = (∑fy)/∑f = ( ∑fx)/∑f  -  57  
= mean(x) - 57 .   And   now  for the  most  important part  we note  that  x - mean(x) = y - mean(y) ; so  that    f ( x - mean(x) )^2  = f (y - mean(y) )^2 
= fy^2  - 2f y mean(y) + f(mean(y))^2   , one can not  but notice that with respect to summation over all values of f and y , the mean is constant. So summing we get  ∑ f(x - mean(x) )^2 
= ∑ f (y)^2  - 2 mean(y)∑ fy  + (mean(y))^2 ∑ f   =∑f (y)^2  - 2 (mean(y))^2 ∑f + (mean(y) )^2 ∑f  = ∑f (y)^2  - (mean(y))^2 ∑f  ;  since  (mean(y))^2  ∑f  = ( ( ∑fy) / (∑f ) )^2  ∑ f  = (∑fy) (∑fy)/(∑f) 
=mean(y) ∑fy , we  can  also write  the formula as ∑f (y)^2  -  mean(y) ∑fy  ;  the  rest  follows . I  hope  it  is  clear  to you now.