Question 1012590
I'm thinking you mean
{{{3^(sqrt(21x)) = 12^(sqrt(21))}}}
If so, I think I would take the log of both sides and get
sqrt(21x)*log3 = sqrt(21)*log12 
sqrt(21x)*log3 = sqrt(21)*(log4 + log3)
sqrt(x)*log3 = log4 + log3 
sqrt(x) = (log4 + log3) / log3
so that
x = [(log4 + log3) / log3]^2 = 5.116