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Pseudo Statistic

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I'm having problems with some Physics problems; among them:

9. At the top of a cliff a 100 m high, a rock is thrown upward with velocity 15 m/s. How much later should a second rock be dropped from rest so both stones arrive simultaneously at the bottom of the cliff?

a. 5.05 s

b. 3.76 s

c. 2.67 s

d. 1.78 s

2. A 50-gram superball traveling at 25 m/s is bounced off a brick wall and rebounds at 22 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 3.5 ms, what is the average acceleration of the ball during this time interval?

a. 13,428 m/s2

b. 6,715 m/s2

c. 857 m/s2

d. 20 m/s2

For 9), I thought I'd use v = u + at and find 2t after solving:

0 = 15 - 9.8t for t, and that would give me the answer... but I'm wrong.

Anyone know how I can go about this one?

And for 2), I really don't know what to do...

Thanks for any help.