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How do you determine whether a relationship represented as as graph is linear or nonlinear

Andy wants to find the distance d across a river. he located points as shown in the figure, then use similar triangles to find that d = 223.5 feet. how can you tell without calculating that he must be wrong? tell what you think he did wrong and correct his error. round your answer to the nearest tenth if necessary. andy must have used the wrong . the distance d is actually feet.

The sum of 15 and two times a number is 29 . what is the number