如图:在平行六面体\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,\(M\)为\(A_{1}C_{1}\),\(B_{1}D_{1}\)的交点\(.\)若\( \overrightarrow{AB}= \overrightarrow{a}\),\( \overrightarrow{AD}= \overrightarrow{b}\),\( \overrightarrow{AA_{1}}= \overrightarrow{c}\),则向量\( \overrightarrow{BM}=(\) \()\)