A unit vector in the direction of a would be the vector a multiplied by a factor such that the length of vector a is unity.
The magnitude of a is given by:
|a|
=sqrt(4^2+3^2+2^2) [ I got lazy instead of writing (-4)^2+(-3)^2+2^2]
=sqrt(29).
So the univector is a/sqrt(29), or (-4/sqrt(29),-3/sqrt(29), 2/sqrt(29), or simply (-4,-3,2)/sqrt(29).