6. 若向量a→\overrightarrow{a}a，b→\overrightarrow{b}b满足∣a→∣=3\vert \overrightarrow{a}\vert =3∣a∣=3，∣a→−b→∣=5\vert \overrightarrow{a}-\overrightarrow{b}\vert =5∣a−b∣=5，a→⋅b→=1\overrightarrow{a}\cdot \overrightarrow{b}=1a⋅b=1，则∣b→∣=\vert \overrightarrow{b}\vert =∣b∣= ．