5/x any values of variable for ehich each rational expression is undefined

Pls! does anybody know a shortcut for answering these types of questions in the future?

The swimming pool is open when the high temperature is higher than 20∘c. lainey tried to swim on monday and thursday (which was 33 days later). the pool was open on monday, but it was closed on thursday. the high temperature was 30 ∘c on monday, but decreased at a constant rate in the next 33 days. write an inequality to determine the rate of temperature decrease in degrees celsius per day, d, from monday to thursday.

Find the domain and range of the following function f(x) = 51x - 21+ 4