If you try to evaluate limx$3$ x$^2-$x$-6/$x$-3$ by plugging in $3$ for x$,$ you get $0$ on top and bottom. This does not give us any information about the limit$.$ However, if you factor and reduce, you get

$$

Then, you can plug in $3$ to get$$limx$3$x$2-$x$-6/$x$-3=$limx$3$ x$+2=$