Determine the value of x will be marked BRAINLIEST

How do you use the elimination method for this question? explain, because i really want to understand!

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?

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.