First of all, we need to figure the TOTAL distance the elevator will travel from below street level to above street level. We know it's 15 meters below street level and 51 meters above street level, so let's add those two numbers.
51 + 15 = 66.
Now, we know the total distance the elevator will travel. We need to divide the distance that will be traveled by the rate of 2 meters per second.
66 / 2 = 33
There's your answer. The elevator will take 33 seconds to travel from the lowest story below street level to the highest story above street level.
A person could ride the elevator for a total of 33 seconds.
Hope this answer helped. :)