SOLUTION: Hello! I have a word problem and I have no idea how to solve it, if you could solve it and show me the steps: A rock is thrown from a top of the tall building. The distance,in fee

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Question 553584: Hello! I have a word problem and I have no idea how to solve it, if you could solve it and show me the steps:
A rock is thrown from a top of the tall building. The distance,in feet, between the rock and the ground t seconds after it is thrown is given by d= -16t^2-4t+412. How long after the rock is thrown is it 400 feet from the ground? (answer should be in decimal form)
Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
Start with:
d+=+-16t%5E2-4t%2B412 You are being asked to solve this equation for t when d (distance from the ground) is 400, so you substitute d = 400.
400+=+-16t%5E2-4t%2B412 Subtract 400 from both sides.
0+=+-16t%5E2-4t%2B12 Factor -4 to make it a little easier to solve.
-4%284t%5E2%2Bt-3%29+=+0 Factor the trinomial.
4t%5E2%2Bt-3+=+%284t-3%29%28t%2B1%29 so now you have:
-4%284t-3%29%28t%2B1%29+=+0 Apply the zero product rule.
-4+=+0 or 4t-3+=+0 or t%2B1+=+0
-4+=+0 No way!
4t-3+=+0 so 4t+=+3 and t+=+3%2F4 or highlight%28t+=+0.75%29
t%2B1+=+0 so t+=+-1 Discard this solution as the time must be a positive quantity.
The rock will be 400 feet from the ground in 0.75 seconds.