Question 386394
We must first choose a frame of reference. Here, I let the acceleration due to gravity be negative, and the ground is at 0 meters.

Now we have {{{0 = (1/2)(-9.8t^2) - 20t + 380}}} (Note that all units must be consistent, so I ignored them for now)


{{{0 = -4.9t^2 - 20t + 380}}}


Now, just solve using the quadratic formula:


{{{t = (20 +- sqrt(7848))/(-9.8)}}} = 6.99887...seconds (quite close to 7 s)