4. After 1 minute, a file Portia is downloading from the internet is at 0 kilobytes downloaded. After 2 minutes, the file is at 3 kilobytes downloaded. After 3 minutes, the file is at 8 kilobytes
downloaded. After 4 minutes, the file is at 15 kilobytes downloaded. Write an expression to model
the amount of the file downloaded. Explain your reasoning.