Answer
$x^2+6x+9$
Work Step by Step
The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$.
The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$.
Hence here: $(x+3)^2=x^2+2\cdot3x+3^2=x^2+6x+9$
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