Identify the horizontal and vertical asymptotes, if any, of the given function. $$f(x) = \frac{3 x^2 + 12 x + 9}{7 x^2 - 7 x - 84}$$
Separate multiple answers by commas. Enter DNE if an asymptote does not exist.

a) Horizontal asymptote(s):      $y =$
b) Vertical asymptote(s):      $x =$