How do you calculate the angles between 2 vectors

The main question has 3 parts to it, which included;
Let P(1, –2), Q(–2, 1), R(3, 5) be three points

The already completed questions are:
1. Calculate the vectors PQ and PR ( --->)
2. Draw a diagram indicating the points P,Q and R, and PR and PR.

The question I need help is with the 3rd question;
3. Calculate the angle between PQ and PR (----->)

I am struggling with this;
Would be great if someone could work it out or atleast explain step by step on how to do this.

Thanks in advance

Hint: Use dot product.
PQ ⋅ PR = |PQ||PR|cosθ

Can you get it now?