Use a quadractic regression equation to predict population.

I was given the population of Las Vegas for every 10 years starting at 1900 and ending in 1990. From these 10 data points I created a table and Graph with my Ti-83.

I found the quadratic regression equation. P =population t=time since 1900

P(t)=53.4804t^2-2217.5496t+12820.6454

I also found the x intercepts and the vertex.

X=34.5202

X=6.9445

The vertex

X=20.7324

Y=12820.6454

Now this is the part where I am stuck. My teacher wants me use to use the above information to determine when will (was) the population of Las Vegas be 1 million people, when was (will) the population be zero? He also said some answers might not be possible with my data(Las Vegas pop)

So I guess that I need to substitute 1 million into P of my equation and solve for t. Is this right ? If so how would I go about that? or is it something else?

I appreciate any help http://www.sosmath.com/CBB/images/smiles/icon_lol.gif and let me know if you need to me to clarify anything...I'm new.