Find a possible formula for the rational function with the following properties: zeros at x=3 and x=6, vertical asymptote of x=7, horizontal asymptote of y=-3.
a) (x-3)(x-6)/(x-7)²
b) (x-3)(x-6)/(x-7)
c) (x-3)(x-6)/(x-7)(x+3)
d) (x-3)(x-6)/(x-7)(x+6)