Hello u2_waYou are right: if and are non-zero, non-parallel vectors then .

If and are parallel, then, for some :In which case, all you can say is that, for this value of .

i.e. (note: a zero vector)

If , and , then and can take any value.

Similarly if , and , then and can take any value.

I think that just about covers all possible cases.

Grandad