good question
acceleration is constant because it is gravitational
acceleration... constant throughout the projectile motion
speed at highest is 0 because it is the turning point. Imagine,
you move upwards, then slow down, then 0, then start to accelerate
downwards. If it cannot reach 0, then how to turn?
speed at the end is 8 m/s as well. Note that this is speed; if
it is velocity, it will be 8m/s downwards, and initial vel will be
8m/s upwards. The reason for the same speed is because of
conservation of energy. At first, the k.e. gets converted to g.p.e.
as it rises, till the turning point, it starts coming down. As it
comes down, the g.p.e. starts changing back to k.e. At every point,
k.e. + g.p.e. is a constant.