Question 45581
Determine the domain of the following:
f(x)=2x-1/3x+7

Now you have a denominator.  It cannot be zero.
So you set it equal to zero and find out the 
value x cannot take, as follows:

3x+7=0
x=-7/3

DOMAIN: All Real Numbers except x=-7/3

Cheers,
Stan H.