I assume a is meant to be: a = 2i - 6j + k
with b = -3j - 2k
To add them you just add corresponding coefficients of i, j and k to get:
c = 2i - 9j - k
To get the unit vector in this direction divide each coefficient by the size of c
|c| = sqrt(22 + (-9)2 + (-1)2) → sqrt(86)
c = (2i - 9j - k)/sqrt(86)
.