Answer
$W=-2.55\times10^6J$
Work Step by Step
We have $F=3\times10^3N$ and $s=850m$
The force is applied to stop the truck, so $\vec{F}$ and $\vec{s}$ are in opposite direction, meaning $\theta=180^o$
The work done by $F$ is $$W=(F\cos180)s=-2.55\times10^6J$$
As we can see, the work is negative, because the force applied is opposite in direction from the displacement.
