Electrical Circuit: Finding Max/Min Voltage in 5 min

[Koyuki]

The problem is:
"In an electrical circuit, the voltage, V volts, as a function of time, t minutes, is given by V = 12 - 9t + 2t^2. Determine the greatest and least values of voltage during the first 5 min. When do these values occur?"

The answers given are exact.
Least: 1.875 V @ 2.25 min
Most: 17 V @ 5 min

Well I know how to get the max, but the min?

Thanks!

 

[hypermorphism]

...
Well I know how to get the max, but the min?
...

The graph is a parabola. You can find the vertex if you remember how, or you can find where the derivative is 0.