8. The solution to the equation $3 \times 16^{2}+2 \times 81^{x}=5 \times 36^{x}$ is
8.0 or $\frac{1}{2}$.

The original equation can be transformed into
$$
3\left[\left(\frac{4}{9}\right)^{x}\right]^{2}-5\left(\frac{4}{9}\right)^{x}+2=0 \text {. }
$$

Let $\left(\frac{4}{9}\right)^{x}=t$. Then
$$
\begin{array}{l}
3 t^{2}-5 t+2=0 \Rightarrow t_{1}=1, t_{2}=\frac{2}{3} \\
\Rightarrow\left(\frac{4}{9}\right)^{x}=1 \text { or } \frac{2}{3} \Rightarrow x=0 \text { or } \frac{1}{2} .
\end{array}
$$