Evaluate each of the limits:

$(2)$

a$)\underset{x-\frac{2}{3}}{lim}\frac{9x^2-4}{3x2}$

$(2)$

b$)\underset{n1}{lim}\frac{n-1}{n^2-1}$

$(2)$

c$)\underset{t}{lim}\frac{3t^2-2x^2-9}{t^4}$