Ok, we have 1 horizontal asymptote and 1 vertical asymptote, so this is a rectangular hyperbola, in the form:

The asymptotes, and thus the function, have shifted up 21 (=b) and shifted left -7 (=a).

Thus,

To find c, simply substitute in a known coordinate: (6,0) or (0,-18).

Substituting in (6,0), we get

Thus the equation is: