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How can you tell when x and y are not directly proportional?

Prove that the value of the expression 7^8–7^7+7^6 is divisible by 43.

Create a question with this scenario you could ask that could be answered only by graphing or using logarithm. david estimated he had about 20 fish in his pond. a year later, there were about 1.5 times as many fish. the year after that, the number of fish increased by a factor of 1.5 again. the number of fish is modeled by f(x)=20(1.5)^x.