Well, as I see it, there are four ways to make that expression zero.
1. . Probably not acceptable, I'm guessing.
2. . See above.
3. is a constant, derived as follows:
4. Finally, you could have defined as a function of thus:
Since there are no other variables, and all of the variables show up in the equation in a linear fashion, I see no other ways to make the expression zero.
Hope this helps.