Help Me, please.

4a^2 + 20ab +25b^2 + 4ac - 10bc + c^2

The factorization of  this isn't easy to see, Manuel....it is this :

(2a + 5b - c)  ( 2a + 5b - c )

We can can check this by  distibuting each term in the first set of parentheses over the terms in the second set of parentheses

4a^2 + 10ab - 2ac

         + 10ab           + 25b^2 - 5bc

                    - 2ac                 - 5bc   + c^2 

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4a^2  + 20ab - 4ac + 25b^2 - 10bc + c^2

So....the factorization is

( 2a + 5b + c)^2

And  taking both roots of this we have

±√(2a + 5b - c )