设\( \overrightarrow{a}\),\( \overrightarrow{b}\)是两个不共线的向量,若\( \overrightarrow{AB}=2 \overrightarrow{a}+k \overrightarrow{b}\),\( \overrightarrow{CB}= \overrightarrow{a}+3 \overrightarrow{b}\),\( \overrightarrow{CD}=2 \overrightarrow{a}- \overrightarrow{b}\),且\(A\)、\(B\)、\(D\)三点共线,则\(k=\) ______ .