30-x is a power of 3. How many powers of 3 are less than 30? there are 3^1, 3^2, 3^3. we immediately see that the answer is 3^3=27 because 27+3=30. we also immediately see that this is the only solution because exponential of x and x do not have the same growth rate, so if x=3 is a solution this solution is unique.
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