[please help]
Peter is conducting a chemical experiment in a controlled environment. The initial temperature of the chemical solution was 20°C. The temperature of the solution was 1.5 times the initial temperature after 3 minutes. The temperature of the solution continued to increase at this rate. This situation is graphed below.

Let x represent the number of minutes passed after the start of the experiment, and let f(x) represent the temperature of the solution.

Once the temperature reached 45°C, the experiment was stopped. Over which interval did Peter conduct the experiment?