\[3,45 \cdot (2,08 - k) = 6,21\]
\[2,08 - k = 6,21\ :3,45\]
\[2,08 - k = 1,8\]
\[k = 2,08 - 1,8\]
\[k = 0,28.\]
\[Ответ:\ \ k = 0,28.\]