The answer is four hours, because if you take the 12 meters you need and devide it by the three meters you get per hour you get four. Therefore the answer is four hours.
We know the rate of the change, 3 meters per hour. And we want to know how long it will take to reach 12 meters. Say we need to reach 6 meters instead 12. How long will that take? 2 hours! Because we divide 6 m by 3 m/h. Now let us do the same thing will 12 meters. 12 m / 3 m/h = 4 hours It will take 4 hours to reach the 12 meters height.