Đáp án:
$\begin{array}{l}
a)\left( {{2^{2020}}.32 - {2^{2020}}.3} \right):{2^{2021}}\\
= \left( {32 - 3} \right){.2^{2020}}:{2^{2021}}\\
= \frac{{29}}{2}\\
b)\left( {{3^{1999}}.45 - {3^{2000}}.21} \right):{3^{2001}}\\
= \left( {{3^{1999}}{{.3}^2}.5 - {3^{2000}}.3.7} \right):{3^{2001}}\\
= \left( {{3^{2001}}.5 - {3^{2001}}.7} \right):{3^{2001}}\\
= \left( { - 2} \right){.3^{2001}}:{3^{2001}}\\
= - 2
\end{array}$
