Kinematics of a particle

This time I'm having a bit of trouble with the mechanical side of mathematics.
Here is my problem:

A stone is catapulted vertically upwards with a velocity of 25ms^1 from a point 2m above the ground. Find (a) its velocity when it hits the ground (b) the total time it is in the air.

Now I can do (a) using
v^2 = u^2 + 2as
However when applying this formula:
s = ut + 0.5 X at^2
to (b) I just can't seem to get the correct answer (2.04s)

Any help is highly appreciated (would ask the teacher but have just broken up for 2 weeks).