The equation to be used here would be:
v = u - gt
where u is the initial speed, g is the acceleration due to gravity and t is the time it takes
a)
u = 28 m/s, g = 9.8 m/s^2, t = 2s
v = 28 m/s - 9.8 m/s^2 2s = 8.4 m/s
after 2 s the lava bomb is still travelling upwards
b)
u = 28 m/s
g = 9.8 m/s^2 t = 3s
v = 28 m/s - 9.8 m/s^2 3s = -1.4 m/s after 3s the lave bomb is travelling downwards
