Hi, Fibonacci. What have you tried so far? Do you know how to find the magnitude (norm) of a vector? If so, you can use the conditions given to form a set of equations.
Let me get you started:
We are told that , where is a unit vector, which means its magnitude is 1, and is positive.
Since , we have
Square both sides:
And since , you can discard the negative solution. Does that help?