How to find the angle between two vectors on a canvas?

Question

JavaScript

Hello. I've always had a little trouble with math.
I have the following task:
There are two vectors on the canvas and I need to calculate the angle between them.

What is the formula for how to calculate this angle?

Answer 1

cos(alfa) = (a * b) / (|a| * |b|)
Vectors, I hope you know how to multiply.

Answer 2

https://jsfiddle.net/pnur201L/1/