To complete the square, we want to write the left side in the form $(x+D)^2$.
The general form of a perfect square trinomial is $(x+D)^2 = x^2 + 2Dx + D^2$.
Comparing $x^2 - 8x$ with $x^2 + 2Dx$, we have $2D = -8$, which means $D = -4$.
So, $D^2 = (-4)^2 = 16$.
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