Time taken to reach the water's surface

[brunettegurl]

Homework Statement

A thin spherical shell with a mass of 3.30 kg and a diameter of 0.225 m is filled with helium (density = 0.180 kg/m3). It is then released from rest on the bottom of a pool of water that is 3.77 m deep. Neglecting frictional effects, determine the value of that acceleration.
Correct, computer gets: 7.91E+00 m/s^2

How long does it take for the top of the shell to reach the water's surface?

Homework Equations

The Attempt at a Solution

so i know that to reach the top of the container it would travel 3.77 m and that vinitial = 0m/s and we know that acceleration is 7.91m/s^2 so i used the equation

$$\Delta$$d= Vi*t+0.5at²
-3.77m=0+0.5(7.91)t²
and when i solve this i get a negative under the root which i know isn't right..pls help

 

[cepheid]

In doing this problem, you've implicitly defined the "up" direction as being positive. So why is your delta d negative?

 

[brunettegurl]

even if i put my delta d as positive i still get the wrong answer