Question 279986
Rational expressions are defined over the set of all real numbers EXCEPT you can not divide by zero. So set your denominator equal to zero and solve. 

It takes a bit more math to understand why we can't divide by zero, but if you look at a graph of the function f(x)=1/x, you will see that, if you get close to 0 from the left and right of the y-axis, the function grows quickly in opposite directions.