Answer
$(x-5)^{2}$
Work Step by Step
$(x+5)^{2}-20(x+5)+100\qquad$...recognize a perfect square trinomial.
$(A\pm B)^{2}=A^{2}\pm 2AB+B^{2}$; $A=(x+5),\ B=10$
$=(x+5-10)^{2}$
$=(x-5)^{2}$
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