Adrian and Vanessa are learning about microalgae as a biofuel in their chemistry class. They are writing a lab report and need an exponential equation that models the growth of the unicellular algae they observed. They started with 2 cells of algae. 1 hour later, there were 6 cells. If there were 18 cells of algae 2 hours from the start of their observation, what function can they use to model the growth of the algae?