For which values of x is x² - 9 < 0?

Factor: $x^2-9=(x-3)(x+3)$. The product is negative when the factors have opposite signs, which happens between the roots $-3$ and $3$. So $-3<x<3$, choice F. Test $x=0$: $-9<0$ is true. Test $x=4$: $7<0$ is false. Choice G ($x<-3$ or $x>3$) is the region where the product is positive, which is the solution of $x^2-9>0$. Choices H ($x<3$) and J ($x>-3$) each include one outside region, such as $x=-5$ where $25-9>0$, or $x=5$.