Check the picture below, so that happens when h = 0.
[tex]\stackrel{h}{0}~~ = ~~-5t^2+15t\implies 0=-5t(t-3)\implies 0=t(t-3)\implies t= \begin{cases} 0\\ 3~\checkmark \end{cases}[/tex]
now, notice, 0 is also a solution, which means at 0 seconds the ball was touching the ground, well, it was kicked "from" the ground, so at 0 seconds it was initially on the ground anyway.
