Arrow goes into block distance, relative, problem, process

amy

1. The problem statement, all variables and given/known data
An arrow is fired with a speed of 22.0 m/s at a block of Styrofoam resting on a smooth surface. The arrow penetrates a certain distance into the block before coming to rest relative to it. During this process the arrow's deceleration has a magnitude of 1570 m/s^2 and the block's acceleration has a magnitude of 450 m/s^2.
a)How long does it take for the arrow to stop moving with respect to the block?
b)What is the common speed of the arrow and block when this happens?
c)How far into the block does the arrow penetrate?2. Relevant equations
I tried doing a= v/t. to find the time. but the answer wasnt right. i really dont know where to start for it.3. The attempt at a solution
I need help on parts a and b. i really have no ideas what to do.  

    i just need a formula to get going.  

    For arrow, v= u -a1*t. For block, v = 0 + a2*t. You find v and t. Enough?