How do you find a unit vector b perpendicular to another unit normal vector, which is #4/5hati + 2/5hatj - 3/5hatk#?
There are at least two fundamental problems with this question.
There's something wrong with this question, since the given vector is not a unit vector, though it says it is.
Perhaps the first step is to make it one by normalising it.
The other problem, though, is that in 3D space there is literally an infinite number of unit vectors perpendicular to this vector, since making it perpendicular only constrains two dimensions.
If you have written the question out incorrectly, please correct it.
If you have written it out correctly, please challenge your teacher or textbook. ;-)