i can;t figure out how to work this problem.

A waterfall has a height of 1500 feet. A pebble is thrown upward from the top of the falls with an initial velocity of 16 feet per second. Theheight, h, of the pebble after t seconds is given by the equation h equals negative 16 t squared plus 16 t plus 1500

h=16t2+16t+1500. How long after the pebble is thrown will it hit theground?

The pebble will hit the ground about ___ seconds after it is thrown.

thank you!