ASIDE-----------------------------
Notice the following:
k + k + k = 3k
w² + w² + w² = 3w²
xy + xy + xy = 3xy
Likewise, 3^x + 3^x + 3^x = 3(3^x)
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Now let's answer the question....
Given: \(3^x+3^x+3^x=3^{21}\)
Simplify left side: \(3(3^x)=3^{21}\)
Rewrite the first \(3\) as follows: \((3^1)(3^x)=3^{21}\)
Apply the Product Law to simplify the left side: \(3^{x+1}=3^{21}\)
Now that we have the same base on both sides, we can conclude that the exponents are equal: \(x+1 = 21\)
Solve to get: \(x = 20\)

Answer: D

Cheers,
Brent