#### Answer

a. 2800 J.
b. 2100 J.

#### Work Step by Step

The work done is the signed area under the force vs position graph.
a. From 0 to 10.0 m, the shape is a trapezoid, with an area of $W_1 = (400 N)\frac{1}{2}(10 m + 4 m) = 2800 J$.
b. From 10.0 to 15.0 m, the shape is also a trapezoid, but the force is negative. The work done is $W_2 = (-200 N)\frac{1}{2}(5 m + 2 m) = -700 J$.
The net work from 0 to 15.0 m is 2100 J.