What is the factored form of this expression?

If x2 is $x^2$ then use the quadratic formula.

 $x^2+9x+16=0$

$x = {-9 \pm \sqrt{9^2-4 \cdot 1 \cdot 16} \over 2 \cdot 1}$

$x = {-9 \pm \sqrt{81-64} \over 2}$

$x = {-9 \pm \sqrt{17} \over 2}$

Since this is irrational, the expression cannot be factored.