Question: How to find the angle of two vectors?

I have three vectors a, b, c. Knowing that the scalar of product a . b = 1, a. c = 1, b.c=1 and modul of a equal to 1, modul of b equal to 4 and modul of c equal to 4. Now I consider two vectors u = -6*a + b + c and v = 3*a -4*b + c. How to find the angle of two vectors u and v?

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