B2. Expand the expression $4^{2 x+1}+4^{x+2}-84$.
B2. Introduce a new variable $a=4^{x}$. We get

$$
\begin{aligned}
& 4^{2 x+1}+4^{x+2}-84=4 \cdot a^{2}+4^{2} \cdot a-4 \cdot 21= \\
& \quad=4\left(a^{2}+4 a-21\right)=4(a+7)(a-3)
\end{aligned}
$$

or $4^{2 x+1}+4^{x+2}-84=4\left(4^{x}+7\right)\left(4^{x}-3\right)$.