Help with finding quadratic equation.

hello, totally new here (Hi)

and i'm having trouble deriving the equation out of this question...

so here's the question:

Alex was practising his 10m platform dive (i'm assuming that it means, he's above the water by 10m?). Because of gravity, the relation between his height, *h*, in metres, and the time, *t*, in seconds, after he dives is quadratic. If Alex reached a maximum height of 11.225m after 0.5 seconds, how long was he above the water after he dove?

So.. i think i'm supposed to find the 2 roots to solve?

thank you guys ! (Talking)