Solution
Basically the domain represent all the possible values that the function can assume on the x axis
And the real numbers represent all the possible numbers from - infinity to infinity
For example:
f(x)= 2x+1
The domain is all the real numbers since the function is defined for all the possible values of x
In the other hand:
For this new function the domain is all the real numbers except x=1 since we can' divide by 0