Hello darklight

Welcome to Math Help Forum!Your first question asks how you should write, whenOf the two answers you give, the one with the sign is correct:

since you have taken the sign into account by mutliplying by to get .

Your question seems to ask: How do I solve

If so, it doesn't make sense, since is a scalar and and are vectors. You can't add vectors and scalars like this.

What did you really mean?

Grandad