Câu trả lời:

\(3^x+3^{x+2}=810\)
\(\Rightarrow3^x+3^x.3^2=810\)
\(\Rightarrow3^x\left(1+3^2\right)=810\)
\(\Rightarrow3^x.10=810\)
\(\Rightarrow3^x=810:10\)
\(\Rightarrow3^x=81=3^4\)
\(\Rightarrow x=4\)
KL: \(x=4\)