1. $\sin{3x} + \sin{2x} = 0 \\
\Leftrightarrow \sin{3x} = - \sin{2x} \\
\Leftrightarrow \sin{3x} = \sin{(-2x)} \\
\Leftrightarrow
\left[\begin{matrix}
3x = -2x + k2 \pi \\
3x = \pi - (-2x) + k 2 \pi
\end{matrix}\right.
$
2. C1: $\cos{4x} + \cos{2x} = 0 \\
\Leftrightarrow \cos{4x} = - \cos{2x} \\
\Leftrightarrow \cos{4x} = \cos{( \pi - 2x)} \\
\Leftrightarrow
\left[\begin{matrix}
4x = \pi -2x + k2 \pi \\
4x = - ( \pi - 2x) + k 2 \pi
\end{matrix}\right.
$
C2: $\cos{4x} + \cos{2x} = 0 \\
\Leftrightarrow 2 \cos{3x} \cos{x} = 0 \\
\Leftrightarrow
\left[\begin{matrix}
\cos{3x} = 0 \\
\cos{x} = 0
\end{matrix}\right. $