If a is a UNIT vector, what does tha make ?
If a and b have a 60º angle between them, what does tha make ? Something to do with a cosine.
Well,
expand by respective dot product (6a+b)(a-2b)
Pretty much like the normal numbers expansion ( Keep it ordered though )
Then a(dot)a=1
b(dot)b=1
a(dot)b=lengtha * lengthb * cos60
but a, b unit vectors with 1 unit lengths each
Proceed ...
Good Luck.
PS: Wherever you come to let us say
6a(dot)b =6(a(dot)b) can take the constant coeff. out,
a(dot)b=[b (dot) a]=you know what !