已知向量\( \overset{→}{AB} =(1,5,-2)\)，\( \overset{→}{BC} =(3,1,2)\)，\( \overset{→}{DE} =(x,-3,6).\)若\(DE/\!/\)平面\(ABC\)，则\(x\)的值是\((\)　　\()\)