Question

Find angles between a vector and b vector if a vector + b vector + c vector = 0, a vector magnitude = 3, b vector magnitude = 5 and c vector magnitude = 7

Answer 1

clearly, a,b,c form a triangle.
The law of cosines says that the angle θ is given by

3^2 + 5^2 - 2*3*5 cosθ = 7^2

More likely, you want pi-θ