Undefined

For the following problems, for what value(s) of x is each function undefined?

1. $f(x)=\dfrac{1}{8x}$

2. $f(x)=\dfrac{3x}{x-16}$

3. $f(x)=\dfrac{x+12}{x^2+7x+6}$

4. $f(x)=\dfrac{1}{\dfrac{x}{7}-3}$

5. $f(x)=\dfrac{82}{12x^2+31x+7}$

6. $f(x)=\dfrac{2x}{x^3-27}$