If three vectors $\overline{\mathrm{a}}, \overline{\mathrm{b}}, \overline{\mathrm{c}}$ are such taht $\overline{\mathrm{a}}\ne \overline{0}$ and $\overline{\mathrm{a}}\times \overline{\mathrm{b}}=2\left(\overline{\mathrm{a}}\times \overline{\mathrm{c}}\right), \left|\overline{\mathrm{a}}\right|=\left|\overline{\mathrm{c}}\right|=1, \left|\overline{\mathrm{b}}\right|=4$ and the angle between $\overline{\mathrm{b}} \mathrm{and} \overline{\mathrm{c}} \mathrm{is} {\cos }^{-1}\left[\frac{1}{4}\right]$, then $\overline{\mathrm{b}}-2\overline{\mathrm{c}}=\mathrm{\lambda }\overline{\mathrm{a}} \mathrm{where} \mathrm{\lambda }=$