How do you find the angle between two vectors?

Using cosine rule a.b = |a||b|cos(theta) and the example vectors a = (1,2,3) and b = (3,2,1),First explain why the cosine rule exists, use a geometric proof of the cosine rule. Show the magintude of a=b=root(14) and a.b = 10, show cos(theta) = 5/7