What information would be included on a nautical chart of a harbor?

Which geometric principle is used to justify the construction below?

Need plz been trying to do it for a long time and still haven’t succeeded

Will mark brainliest for correct answer! i have been on this problem for an hour and can't find the answer. i'm begging here! incase you cant read the instructions in the picture because of too much or too little lighting: note: enter your answer and show all the steps that you use to solve this problem in the space provided. write a list of steps that are needed to find the measure of b. provide all steps as clear as possible, !