[SOLVED] Calculating units of speed / error?

An avalanche sliding down a mountain has a constant acceleration of 3.0m/s^2 [W]. It takes 6.0s to cover a displacement off 78m[W].

a) Calculate its initial velocity.

G:

Acceleration = 3.0m/s^2

Time = 6.0s

Displacement = 78m [W]

R:

Velocity1

A:

d = v1(t) + 1/2a(t)^2

...Isolating Variable...

(2d - a(t)^2) / 2t = v1

S:

(2(78m[W]) - 3.0m/s^2[W](6.0s)^2) / 2(6.0s) = v1

(156m[W] - 3.0m/s^2[W](36s)) / 12s = v1

(156m[W] - 108m/s^3[W]) / 12s = v1

This is the problem I'm having. I am not able to subtract the different units of speed. My textbook says that the answer to this question is 4.0m/s[W]. If I were to subtract 108 from 156 and divide by 12, I would get 4.0 correct, but no correct units/direction. Where was my error here? I usually get a little confused when I divide s^2 by s or something similar.