$[10$ pts each$]$ Evaluate the following limits$.$

$($a$)\underset{x3}{lim}\frac{3x1}{x^2-5x-7}=$

$($b$)\underset{x{1}^{}}{lim}\frac{x1}{x-1}=$

$($c$)\underset{x{1}^{-}}{lim}\frac{2x3}{(x-1{)}^3}=$