[tex]\mathrm{If\:}f\left(x\right)=g\left(x\right)\mathrm{,\:then\:}\ln \left(f\left(x\right)\right)=\ln \left(g\left(x\right)\right) \ \textgreater \ \ln \left(6^x\right)=\ln \left(21\right)[/tex]
[tex]\mathrm{Apply\:log\:rule}:\ \log _a\left(x^b\right)=b\cdot \log _a\left(x\right) \ \textgreater \ \ln \left(6^x\right)=x\ln \left(6\right) [/tex]
[tex]x\ln \left(6\right)=\ln \left(21\right) \ \textgreater \ \mathrm{Divide\:both\:sides\:by\:}\ln \left(6\right) \ \textgreater \ \frac{x\ln \left(6\right)}{\ln \left(6\right)}=\frac{\ln \left(21\right)}{\ln \left(6\right)} [/tex]
[tex]Therefore \: x=\frac{\ln \left(21\right)}{\ln \left(6\right)}[/tex]
In your case ln means [tex] \frac{log 21}{log 6} [/tex]