You need to evaluuate the dot product of the given vectors `bar u` and` bar v` , such that:
`bar u*bar v = |bar u|*|bar v|*cos theta`
theta represents the angle between the vectors `bar u,bar v`
The problem provides the magnitude of the vector `bar v` , `|bar v| = 3` and the dot product of the given vectors `bar u` and `bar v` , such that:
`-18 = 3*|bar u|*cos theta => |bar u|*cos theta = -18/3`
`|bar u|*cos theta = -6 => |bar u| = -6/(cos theta)`
Since the length of bar u needs to be a positive value, hence, `-1 < cos theta < 0` .
Hence, evaluating the length of the vector bar u yields that `|bar u| >= 6` .