Answer:
[tex]\large{\boxed{\sf 3x(3x + 1)}[/tex]
Explanation:
[tex]\sf \rightarrow 9x^2 + 3x[/tex]
Rewrite the expression
[tex]\sf \rightarrow 3x\cdot \:3x+3x[/tex]
In this expression, the term 3x is a common factor. So take it out
[tex]\sf \rightarrow 3x(3x + 1)[/tex]
The expression is factored completely
