I'm a bit rusty, how do you calculate these stochastic problems?

So, here we go, "Physics 2," or what? Warm outside, cold inside. What does that mean? Well, nothing at first. The following problem needs to be solved. Approach I know that r_1,2 is the same as r_2 – r_1 and | r | is the same as the square root of r_x² + r_y². So,…