Calculating Angle Between Vectors Using Cosine Law

[Uira]

Hello, I've been trying to solve this problem for hours now, but i keep getting it wrong. Been looking for examples but i don't seem to find one with a good explanation, so any help is appreciated.

The problem:

Consider two displacements, one of magnitude 2.6 and another of magnitude 3.9. What angle between the directions of these two displacements give a resultant displacement of magnitude a) 5.7m b)2.6m c)3.2

If anyone can please help me understand, thank you. Please, don't just post the answer, i want to learn how to solve it.

 

[NascentOxygen]

This is referring to addition of the displacements.

You have a triangle, and know the lengths of the 3 sides. Find the angle/s.

 

[HallsofIvy]

A formula that will help is the cosine law. If a triangle has sides a, b, and c and C is the angle opposite side c, then $c^2= a^2+ b^2- 2ab cos(C)$