actually i need help on this quadratic function

f(x) = -x^{2 }+ 10x

here is my work

f(x) = -x^{2 }+ 10x

f(x) = -x^{2}+ 10x + 25 - 25 ((i divided 10 with 2 and then i squared it to get 25, then i added and subtracted 25 to keep the function even))

f(x) = -1(x - 5)^{2}- 25

here is where i am stuck... how can i factor -x^{2}+ 10x + 25?

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