\input amstex

$$\align
e^{\hbox{\tiny\pmb i}336t}+e^{-\hbox{\tiny\pmb i}336 t}=0\;&\Longleftrightarrow\;e^{\hbox{\tiny\pmb i}336\cdot 2 t}=-1\;\Longleftrightarrow\\
&\Longleftrightarrow 672 t=\frac{\pi}2+2k\pi\;\Longleftrightarrow \fbox{$\displaystyle
t=\frac{\pi}{1344}+\frac{k\pi}{336}$}\text.\\
\endalign$$
\bye