"In an electrical circuit, the voltage, V volts, as a function of time, t minutes, is given by . Determine the greatest and least values of voltage during the first 5 min. When do these values occur?"

The first thing I did was substitute .

Then I go check the answer page to see if I was right. The answer is "Least value: 1.875 V at 2.25 min; greatest value: 17 V at 5 min". How was the first part of the answer obtained? Is completing the square involved? If it is, I already have that done.

I'm guessing that this wasn't needed, as this seems to have no relevance to the aforementioned answer. What needs to be done here? Help would be appreciated.