\[\boxed{\text{6.251.}}\]
\[\textbf{а)}\ 21z = 0,21\]
\[z = 0,21\ :21\]
\[z = 0,01\text{.\ }\]
\[Ответ:0,01.\ \]
\[\textbf{б)}\ 2,64\ :a = 4\]
\[a = 2,64\ :4\]
\[a = 0,66.\ \]
\[Ответ:0,66.\ \]
\[\textbf{в)}\ 317,8\ :(n - 5) = 14\]
\[n - 5 = 317,8\ :14\]
\[n - 5 = 22,7\]
\[n = 22,7 + 5\]
\[n = 27,7.\ \]
\[Ответ:27,7.\ \]
\[\textbf{г)}\ 38 \cdot (x + 1,3) = 64,6\]
\[x + 1,3 = 64,6\ :38\]
\[x + 1,3 = 1,7\]
\[x = 1,7 - 1,3\]
\[x = 0,4.\ \]
\[Ответ:0,4.\]