Precalculus, Chapter 6, 6.3, Section 6.3, Problem 46

For a nonzero vector w a unit vector a with the same direction may be found by the formula
a=(1/||w||)*w.
It has the same direction because it is a w multiplied by a positive number.
It has a magnitude of 1 because
||a||=|1/||w|| |*||w||=(1/||w||)*||w||=1.

In this case ||w||=6 and a=-i. No doubt that ||a||=sqrt((-i)*(-i))=1.