The expression is undefined if
the denominator equals 0 when substituting
x=3.
Expression
A is defined for
x=3 because insertion gives
71 which is a . Let's continue with
B.
Expression
B is not defined for
x=3 since is forbidden. Finally, we will try expression
C.
18−2x28
18−2⋅328
18−2⋅98
18−188
08
Nor is
C defined for when you insert
x=3. In conclusion,
A is defined, and
B and
C are undefined for
x=3.