height = 32*(-1/2)*t^2 +vt + s
v = 0 because we're dropping the shovel, not throwing it down
s = 0 from our starting point.
height = 5000 - 3400 = 1600 feet below the top of the shaft
-1600 = 32*(-1/2)*t^2
-1600 = -16*t^2
100 = t^2
t = 10
10 seconds.