How to use a number line???????????

It is always not possible to determine the domain and range of the function by using the number line. Using a number line would require you to draw out the function first. If you teacher says so, then try and discuss with your teacher that it is cumbersome to use always a number line to determine the domain and range of a function. Also, discuss some examples with her from NCERT where they haven't used the number line. If still she says to use number line, then you should learn this method. Start practicising questions using the number line method. 

 

In a number line method, you need to draw the function on a number line and determine at what points the function is not real or has no value at all i.e. not defined. And that would help you determine the range and domain of the function.