Can some one help me with this math question?

Look at the following graph of the given equation. determine whether the equation is a function. explain why or why not.

Equation for the calculation double 2 and then add 5

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. create a question you could ask that could be answered only by graphing or using a logarithm.