Manipulating Rational Expressions

The expression is undefined if the denominator equals 0 when substituting $x=3.$ Expression A is defined for $x=3$ because insertion gives $\frac{1}{7}$ which is a real number. Let's continue with B. Expression B is not defined for $x=3$ since division by zero is forbidden. Finally, we will try expression C. Nor is C defined for when you insert $x=3$. In conclusion, A is defined, and B and C are undefined for $x=3.$