\[\boxed{\text{5.\ 32}}\]
\[\textbf{а)}\ 9,6\ :3 = 3,6\ :\left( \frac{1}{4}x \right)\]
\[\frac{1}{4}x \cdot 9,6 = 3,6 \cdot 3\]
\[2,4x = 10,8\]
\[x = 108\ :24 = \frac{108}{24} = \frac{9}{2}\]
\[x = 4,5.\]
\[\textbf{б)}\ 6\ :4x = 2,6\ :8\frac{2}{3}\]
\[4x \cdot 2,6 = 6 \cdot 8\frac{2}{3}\]
\[10,4x = 6 \cdot \frac{26}{3}\]
\[10,4x = 52\]
\[x = 520\ :104\]
\[x = 5.\]