Let
\[
\vec a=4\hat i-\hat j+\alpha\hat k
\]
and
\[
\vec b=\hat i+\alpha\hat j-4\hat k
\]
be two vectors. If $\alpha_1,\alpha_2$ ($\alpha_1<\alpha_2$) are two different values of $\alpha$ such that
\[
(\vec a,\vec b)=\cos^{-1}\left(-\frac{2}{7}\right),
\]
then
\[
\alpha_1+2\alpha_2=
\]