from the title of your post I assume that you have to use vectors (If you don't want to use vectors you can solve this problem by using Cosine rule)
1. Calculation of the vectors:
2. The angle between 2 vectors and is calculated by:
Plug in the vectors you know into this formula:
Therefore A = 90° : You are dealing with a right triangle.
3. Hint: Draw a sketch of this triangle to prove the calculations.