check the picture below.
the ball hits the table, when it hits "the ground" or goes all the way back down, at that point, the height of the ball is 0, thus h = 0.
[tex]\bf 0=-4.9t(t-0.42)\implies t=
\begin{cases}
0=-4.9t\\
0=t\\
------\\
0=t-0.42\\
\boxed{0.42=t}
\end{cases}[/tex]
