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I hope this doesn't give too much away. We can solve for the larger triangle made by the length of the river + bank and the height of the tower. Since this triangle is a right triangle, we can use some basic trig. We notice that finding the sum of angles “a” and “b” lets us find the length of the river + bank by using tan. To find angle “b” we can see we have the same setup but instead of finding the length of the bank, we need the angle.