Help Me please nobody in my physics class knows the answer lol?

Jim stands beside a wide river and wonders how wide it is. He spots a large rock on the bank directly across from him. He then walks upstream d = 43 strides and judges that the angle between him and the rock, which he can still see, is now at an angle of 30∘ downstream. Jim measures his stride to be about 0.8 m long
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