SOLUTION: If a stone is tossed from the top of a 170 meter building, the height of the stone as a function of time is given by h(t) = -9.8t^2 – 10t + 170, where t is in seconds, and height

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Question 613929: If a stone is tossed from the top of a 170 meter building, the height of the stone as a function of time is given by h(t) = -9.8t^2 – 10t + 170, where t is in seconds, and height is in meters. After how many seconds will the stone hit the ground? Round to the nearest hundredth’s place; include units in your answer.
please this is the only problem I need help with. I just don't understand!
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
++h%28t%29+=+-9.8t%5E2+-10t+%2B+170+
I'll plot this because the plot shows exactly what the
stone is doing at any time after it's thrown.
+graph%28+400%2C+400%2C+-2%2C+5%2C+-20%2C+250%2C+-9.8x%5E2+-+10x+%2B+170+%29+
You can see that at +t+=+0+, the height, +h%280%29+, is 170.
What you are being asked is " How many seconds have passed
after stone is thrown until the stone hits the ground?"
Note that the stone is not thrown straight up and landing at
the 170 m height of the building. It is landing 170 m below this
at the ground level.
At ground level, +h%28t%29+=+0+, so I can now say
++-9.8t%5E2+-+10t+%2B+170+=+0+
Now I can use the quadratic formula to solve for +t+
t+=+%28-b+%2B-+sqrt%28+b%5E2+-+4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
+a+=+-9.8+
+b+=+-10+
+c+=+170+

+t+=+%28+10+%2B-+sqrt%28+100+%2B+6664+%29%29%2F%28+-19.6+%29+
+t+=+%28+10+%2B-+sqrt%28+6764+%29%29%2F%28+-19.6+%29+
+t+=+%28+10+-+82.244%29%2F%28+-19.6+%29+
+t+=+%28-72.244%29+%2F+%28-19.6%29+
+t+=+3.69+ sec ( rounded off )
The stone hits the ground in 3.69 seconds
Note that I had to use the negative root in the formula
because that gave me (-)/(-) or positive time.