(18 pts) Evaluate the following limits. (a)
\lim_(t->5)(3-\sqrt(t+4))/(t-5)
(b)
\lim_(x->3^(-))(x^(2)-7x+12)/(x^(2)-6x+9)
(c)
\lim_(r->\infty )(\sqrt(r^(2)-9))/(2r-6)