Answer
$(a-1)(a+3)(a+1)^{2}$
Work Step by Step
Think of the equation as $x^2-2x-3$, with $x=a^2+2a$. We then get:
$(a^{2}+2a)^{2}-2(a^{2}+2a)-3=[(a^{2}+2a)-3][(a^{2}+2a)+1]=(a^{2}+2a-3)(a^{2}+2a+1)=(a-1)(a+3)(a+1)^{2}$
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