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.