ACT Test#4 problem 41

In the figure below, a radar screen shows 2 ships. Ship A is located at a distance of 20 nautical miles and bearing 170°, and Ship B is located at a distance of 30 nautical miles and bearing 300°. Which of the following is an expression for the straight-line distance, in nautical miles, between the 2 ships?

(Note: For ABC with side of length a opposite ∠A, side of length b opposite ∠B, and side of B, and side of length c opposite ∠C, the law of c2 = a2 + b2 − 2ab cos,∠C.)
there is also a drawing copy paste this and you can see it

I don’t know how you can get 130º. I know it is 300-170, but why does that happen?
Thank you for your help