if `veca` is a unit vector and `(vecx-veca).(vecx+veca)=8` then `|vecx|`

Answer

`|vec(x)|=3`

Explanation

`(vec(x)-vec(a)).(vec(x)+vec(a))=8implies vec(x).vec(x)-vec(a).vec(a)=8 [ :' vec(x).vec(a)=vec(a).vec(x)]`
`implies |vec(x)|^(2)-|vec(a)|^(2)=8 implies |vec(x)|^(2)=(8+1)=9implies |x|=3`.