Please Help with this Question! I Have No Idea How to Solve it!

So I have this question that I have no idea how to approach,
it was one of my math problems that belong in the category of factoring general quadratics,
but I didn't know how to factor it:
The equation  
$\large{y=-4.9t^2+42t+18.9 }$
describes the height (in meters) of a ball tossed up in the air at 42 meters per second from a height of 18.9 meters from the ground,
as a function of time in seconds.
In how many seconds will the ball hit the ground?

$\text{Hit the ground means $y=0\ !$}$

$\begin{array}{|rcll|} \hline -4.9t^2+42t+18.9 &=& 0 \quad | \quad \cdot(-1) \\ 4.9t^2-42t-18.9 &=& 0 \\\\ t &=& \dfrac{42\pm \sqrt{42^2-4\cdot 4.9 \cdot (-18.9) } } {2\cdot 4.9} \\ t &=& \dfrac{42\pm \sqrt{42^2+370.44 } } {9.8} \\ t &=& \dfrac{42\pm \sqrt{1764+370.44 } } {9.8} \\ t &=& \dfrac{42\pm \sqrt{2134.44 } } {9.8} \\ t &=& \dfrac{42\pm 46.2 } {9.8} \quad | \quad t>0\ !\\ t &=& \dfrac{42{\color{red}+} 46.2 } {9.8} \\\\ t &=& \dfrac{88.2 } {9.8} \\ \mathbf{ t } & \mathbf{=} & \mathbf{9\ \text{seconds} } \\ \hline \end{array}$

In 9 seconds the ball will hit the ground.