Calculating Cannon Ball Barrel Length: Impulse and Velocity

[phizics09]

Homework Statement

The impulse on a human cannon ball is 2.5  103 N·s. The cannon
ball has a mass of 65 kg.

Homework Equations

How long is the barrel of the cannon if the cannon ball leaves the
cannon at 120 km/h?

The Attempt at a Solution

I used the formula J=ma(delta t)
(2.5x 10^3)=(65)(a)(0.2)

to find the acceleration. Then, I used the formula d=v1(t)+(1/2)(a)(t^2) to find the distance, and I got 3.85 m, but the answer at the back is wrong.

Can anybody help me?

 

[ninco]

How did you get the time of 0.2 seconds ?

 

[phizics09]

That's given in part a of the question, which I already got. Here's the entire question:

The impulse on a human cannon ball is 2.5  103 N·s. The cannon
ball has a mass of 65 kg.
a) What force does the cannon exert on the human cannon ball if
it takes 0.2 s for the human cannon ball to leave the cannon?
b) How long is the barrel of the cannon if the cannon ball leaves the
cannon at 120 km/h?