Let $\> f(x)= \begin{cases} x - 1, &x \le 3, \\ 3 x - 7, &x > 3. \end{cases}$.
Find

(a)$\> \displaystyle \lim_{x \rightarrow 3^-} f(x)$:

(b)$\> \displaystyle \lim_{x \rightarrow 3^+} f(x)$:

(c)$\> \displaystyle \lim_{x \rightarrow 3} f(x)$:

Enter I for $\infty$, -I for $-\infty$, and DNE if the limit does not exist.