See this solution/explanation (answer is '7 meters'):
1. According to the condition 'y' means the height, and 'x' - the length from the start of the lift hill.
2. The phrase 'height of 343 meters' means y=343.
3. From the another side y=x³. If to substitute 343 instead of 'y': 343=x³, - it is possible to find the value of 'x'.
x=7 [m] - how far from the start of the lift hill...