This is one of those questions whose answer is hidden by the sloppiness of the wording. You don't really know what the starting speed is at 129 feet. I am going to assume the submarine is constantly moving at the same speed.
Givens
d1 = 129
d2 = 229
t = 3 sec
formula
(d2 - d1) = r * t
solution
(229 - 129) = r * t
100 = r * 3
100/3 = r
33.3 feet/s = r
Caution
if you know what d = vi*t + 1/2*a * t^2 is, you could get this wrong. If you do know the formula, leave a comment below my answer.
