A ball of a mass 100g falls from a height of 3m unto a horizontal surface and bounce to the height of 12m calculate the change in the momentum of the ball when it strikes the surface.
Do you mean bounce to 1.2 meter? I hope.
(1/2) m v^2 at floor = m g h at top
so
v = sqrt (2 g h) (but you knew that)
so
v down = sqrt (2*9.8*3)
v up = sqrt(2*9.8*1.2)
opposite signs so add
change in momentum = m * [ sqrt (2*9.8*3) + sqrt(2*9.8*1.2) ]