Answer
$9a^2+6ab+b^2-6a-2b+1$
Work Step by Step
Use the formula $(a-b)^2=a^2-2ab+b^2$ and treat $3a+b$ as a single term, we have:
$[(3a+b)-1]^2=(3a+b)^2-2(3a+b)+1=9a^2+6ab+b^2-6a-2b+1$
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