hi, i'm starting to look at work on rates of change and was wondering if someone could explain something to me.

the example question goes like this:

the volume, of water is given by . find the rate of change of volume with respect to time after 0.5 hours

it then goes on to and explains that:

when t = 0.5 after 0.5 hours the volume is increasing at a rate of

i don't understand where the power of minus 1 comes from?

can someone please explain this? thankyou