Answer
a) $W=3J$
b) $W=0J$
c) $W=-9J$
Work Step by Step
The work done in each case is the area of the red shape as portrayed in the force-displacement graph.
a) The work done by the force from $s=0$ to $1m$: $$W=\frac{1}{2}\times6N\times1m=3J$$
b) The work done by the force from $s=1$ to $2m$ is $0$ because the area beneath the line in the graph is $0$.
c) The work done by the force from $s=2$ to $4m$: $$W=\frac{1}{2}\times(1m+2m)\times(-6N)=-9J$$
