## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$$(3x+1)^2 .$$
The polynomial $9x^2+6x+1$ is a complete square since it is of the form $a^2+2ab+b^2=(a+b)^2$. Thus, by putting $a=3x$, $b=1$, we have $$9x^2+6x+1=(3x+1)^2 .$$