axb=2axc

b-2c=$\displaystyle \lambda$a

given that |a|=|c|=1 and |b|=4

and the angle betweenbandcis $\displaystyle \arccos \frac{1}{4}$

show that $\displaystyle \lambda =+4 , -4$

For each of these cases find the cosine of the angle betweenaandc.

I don't know where to begin