Question

WHAT WOULD I DO FOR THIS ONE
A small rock sits on the edge of a tall building. A strong wind blows the rock off the edge. The distance, in feet, between the rock and the ground t seconds after the rock leaves the edge is given by d = −16t 2 − 4t + 420. How long after the rock leaves the edge is it 390 feet from the ground?

Answer 1

set = 390 and solve for t

Answer 2

put d = 390 and solve fr t

Answer 3

\[-16t^2-4t+420=390\]
\[-16t^2-4t+60=0\]
etc

Answer 4

so 390=-16t^2-4t+420?

Answer 5

ok thanks!

Answer 6

except that my arithmetic is wrong. it is
\[-16t^2-4t+420=390\]
\[-16t^2-4t+30=0\]

Answer 7

i get 5/4