Can someone help me with both of the or may be one I’m stuck on both of the question if someone could help me I would highly appreciate:)

How do i start to solve? would appreciate a walk-thru! a bird sitting 16ft above the ground in an apple tree dislodges an apple. after how many seconds does the apple land on the ground? (assuming that no branches will interfere with it's fall)

Which expression demonstrates the use of the commutative property of addition in the first step of simplifying the expression (-1+i)+(21+5i)+0

Using only the digits 5, 6, 7, 8, how many different three digit numbers can be formed if no digit is repeated in a number?