So I know the general or standard form is ax+by=c

I converted an answer into standard form, my professor said " ax must be positive whole number" B and C does not matter.

Well in my book the answer made "C" into a whole number as opposed to how it was normally as a fraction.

So does C have to be a whole number, why would the book convert it into a whole number?

Example: I get the answer:

x-2/3y=0 But the book wants us to get : 2x-3y=0 by making "y" a whole number. However, my professor said B, and C does not have to be whole number?)