Higher physics - rearranging prob

[jamespeterstu]

Homework Statement

A nail is fired from a nail gun into a fixed block of wood. The nail has a speed of 380 m s-1 just as it enters the wood.

The nail comes to rest after penetrating 60 mm into the wood.

Find the time taken for the nail to come to rest. Assume that the retarding force on the nail is constant as it penetrates the wood.

http://www.bbc.co.uk/scotland/education/bitesize/higher/img/physics/mech_matt/analyse_motion/example1.gif

Homework Equations

u = 380 m s-1
v = 0
a = ?
t = ?
s = 60 x 10-3 m

The Attempt at a Solution

v2 = u2 + 2as
02 = 3802 + 2a x 60 x 10-3
a = -120333 m s-2

This was from a revision site so they gave the answer, however my problem is that I'm unsure on how to re-arrange the formula properly.

Having just started higher physics (scotland) this doesn't bode too well.

 

[Varnick]

HINT: Use another equation of motion, now that you have a value for a.

 

[jamespeterstu]

HINT: Use another equation of motion, now that you have a value for a.

v = u +at
0= 380 + (-120333)t
t = 0.0003157

Is the answer given, I'm just having trouble getting my head around the re-arranging of formulae :/