Hi Guys. I don't know if this is the right section but I need some explanation on this (plz?)

a) Prove that a=b-kc and b=b+ka ($\displaystyle k\neq 1$) are collinear (a,b,c have those lines up that represent vectors)

b) Find $\displaystyle \alpha$ so that vectors a=2i+3j and b=$\displaystyle \alpha$i-j can be collinear.

I know that these are probably simple but my brain isn't really working well.

Any help greatly appreciated.

Rinor