\(\begin{array}{l}
a)\,\,\,{6^9}:\left( {{{21.6}^8} - {{19.6}^8}} \right)\\
= {6^9}:\left[ {{6^8}.\left( {21 - 19} \right)} \right] = {6^9}:{6^8}:2 = 3.\\
b)\,\,{8^x} + {8^{x + 2}} = 24\\
{2^3}^x + {2^{3\left( {x + 2} \right)}} = 24\\
{2^{3x}} + {2^{6 + 3x}} = 24\\
{2^{3x}} + {64.2^{3x}} = 24\\
{65.2^{3x}} = 24\\
{2^{3x}} = \frac{{24}}{{65}}.
\end{array}\)
Bạn xem lại đề bài câu b nhé,