Simplify and state the values of the variables where the expressions are undefined. \text { a. } \frac{4 x^{2}-16 x+12}{x^{2}-9} \text { b. } \frac{5-2 x}{x+2}+\frac{x^{2}}{x^{2}-4}-5 \text { C }

\frac{3 c}{c^{2}-25} \times \frac{c-5}{6 c^{2}-3 c} \div \frac{2}{c+5} \text { d. } \frac{x-2}{x+5}-\frac{x^{2}-2 x-3}{x^{2}-x-6} \times \frac{x^{2}+2 x}{x^{2}-4 x}