solve for n in the equation 11n and 21n =204n
assuming "and" means +, and the "n" indicates the base, we have
n+1 + 2n+1 = 2n^2+4
that has no integer solutions, so I guess you have somehow mangled the question.
It's not my job to figure out what you mean.
It's your job to say it so clearly you cannot be misunderstood.