Final answer:
The height of the pebble when it was thrown is 15 meters.
Step-by-step explanation:
The height of the pebble at the time it was thrown can be found by substituting x = 0 into the height function h(x) = -5x^2 + 10x + 15.
Therefore, the height of the pebble when it was thrown is h(0) = -5(0)^2 + 10(0) + 15 = 15 meters.