The ball's velocity will be represented by the derivative of its distance function:
[tex]s'(t)=6t^2+6t[/tex]
Now find the times [tex]t[/tex] for which this is equal to 30, i.e. solve
[tex]6t^2+6t=30\implies t^2+t-5=0[/tex]
This has two solutions, [tex]t=-\dfrac{1\pm\sqrt{21}}2[/tex], but only one is positive and falls in the interval [tex][0,3][/tex]. So the velocity reaches 30 cm/s when [tex]t=-\dfrac{1-\sqrt{21}}2\approx1.79[/tex].