To solve this problem you must apply the proccedure shown below:
1. You have the following points given in the problem above:
(–1, 5), (2, 5), (3, –1), and (–1, –1)
2. When you plot each point on a graph, you can see that the figure is a trapezoid, therefore, the area of a trapezoid is:
A=(B+b)h/2
Where B is the larger base, b is the smaller base and h is the height.
3. Base on the information, you have:
B=4 units
b=3 units
h=6 units
4. The area is:
A=21 units²
The answer is the option C: 21 units²