设向量\( \overset{⇀}{a} \)、\( \overset{⇀}{b} \)满\( \overset{⇀}{a} ⋅ \overset{⇀}{b} =-8\)，且向量\( \overset{⇀}{a} \)在向量\( \overset{⇀}{b} \)方向上的投影为\(-3 \sqrt{2} \)，则\(| \overset{⇀}{b} |=\)