Hi AdorableApatosaurus7671,
Thank you for your question :)
When they say that the point of interest is at 15 meters horizontally from the lowest point, you need to calculate de x coordinate of the lowest point (55 meters since the period is 100 m and the function has its max at 5 m), and substract 15 meters. The x coordinate that is found is 40 meters :) I have to say, it is strange that in the text it says 15 meters horizontally from the lowest point and on the graph they directly show 40 m...but anyways, that's what you need to calculate.
Don't hesitate if you have other questions :)
Sandrine
