Umm, this projectile thing is also Mechanics? I remembered it as a Physics thing.

Umm, it is about vectors. So, yes, it must be Mechanics also.

Yes, you are right that the pebble reaches a maximum of 6.4m in part A.

For the part B, it's almost the same as in part A. It is vector thing.

As with vectors, the directions of the flights are very important.

s = Vo*t +(1/2)a*t^2

That's the basic formula for distance or displacement.

In the going vertically up, the acceleration is g and it is negative because the going up is measured as positive.

In the going vertically down, the acceleration is g also....but now it is positive because the going down is measured as positive here.

g is always going down, as you know.

For the going up,

h = 11.2*t -(1/2)(9.8)t^2 -----------(1)

For the going down,

h1 = 0.8*t +(1/2)(9.8)t^2 ----------(2)

h +h1 = 9 m

So, adding (1) and (2)

h +h1 = 12t

t = 9/12 = 0.75 sec ---------answer.

And so,

h = 11.2(0.75) -(1/2)(9.8)(0.75)^2

h = 5.644 m above point A ----------answer.