If the value is:
[tex]\sf{\frac{2}{a+8}}[/tex]
Then it will be undefined when the denominator is equal to 0.
So set the denominator equal to 0
a + 8 = 0
a = -8
So when
[tex]\huge{\boxed{\bf{a=-8}}}[/tex]
then the value will be undefined.
Just a note:
[tex]\sf{\frac{x}{0}=}[/tex] undefined
[tex]\sf{\frac{0}{x}=0}[/tex]
