I have solved an example problem given in my book, but I don't understand why are the second being squared.

$\displaystyle \frac{0 m/s - 30m/s}{3.0s}$ = -10m/s^2

Is it looking like this at some point $\displaystyle \frac{-30 m^2/s^2}{3.0s}$

And if so why are the seconds not canceling?