How do you find the excluded value of #3/(x-5)#?
You need to consider the value of
So, when you have a fraction with unknown in it (in the denominator) always set the denominator equal to zero, solve it as a normal equation and exclude the value(s) of the unknown you find.
For example, in your case:
1) take the denominator:
2) set it equal to zero:
3) solve the equation:
Now you can exclude the result,