Help with partial derivatives for linear least squares

I am refered to this example:

Linear least squares - Wikipedia, the free encyclopedia

If I understand right, the solution of the system

S'b(a)=0

S'a(b)=0

gives the values a=3.5 and b=1.4, for witch the S(a,b) takes the minimum value.

I try to find this results myself, but when I calculate the partial derivatives with respect to a and b of S(a,b), I find:

S'b(a) = 8a+20b-56 (b=constant) ( I thing this is right ) and

S'a(b) = 20a+20b-154 (a=constant) (this dont' fit the results)

probably somewhere I make a mistake ... but where ?(Headbang)

Thanks in advance