The rock is kicked horizontally off the cliff at 20 m/s.
There's no air resistance, and gravity has no effect on horizontal motion. There's no horizontal force acting on the rock to make it move horizontally slower or faster than 20 m/s, so it keeps moving horizontally at 20 m/s.
It's in the air for 7 seconds before it hits the ground. Moving horizontally at 20 m/s for 7 seconds, it sails (20 x 7) = 140 meters horizontally away from the cliff.
