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