$\vec{a}$

$\vec{b}$

O: (2, 3, 1)
a: (3, 1, 2)
b: (-1, 1, 1)
c: (a * b)
A: O + a
B: O + b
u = Vector(O, A)
v = Vector(O, B)
α = Angle(u, v)