Exponential Equation with a Unique Solution

Algebra Level 4

$$r$$ is a positive rational number such that the equation $$r 3^x + 3^{-x} =3$$ has a unique real solution. If $$r = \frac {a}{b}$$ where $$a$$ and $$b$$ are coprime positive integers, what is the value of $$a+b$$?

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