Answer:
70 m/s
Explanation:
"The speed of a tidal wave in meters/second is given by the square root of the product of the acceleration due to gravity on Earth (9.8 meters/second2) and the depth of the ocean in meters":
The sentence above can be interpreted into the equation shown below (letting speed of tidal wave be "s", acceleration due to gravity be "g", and depth be "d"):

Given d = 500 and g = 9.8, we find speed (s):

Hence, the speed is 70 meters per second.