(a) (i)From first equation of motion

Here, we have at maximum height , final velocity, v = 0 and a = -g, and time taken = 3 s

(ii) From second equation of motion

(b) When you're at the centre of the earth, you'd have equal amounts of gravitational force pulling you in all directions from the mass of the earth evenly distributed around you. Thus, the net gravitational force would be zero. This is also known as your weight