To find the rule that represents the area of this triangle, you will need to know the rule for finding area of a triangle.
A = 1/2bh
The base of this triangle is the left vertical line, so to find the distance of this line you will subtract y2 and y1. The height of this triangle starts at (x3, y3), and moves directly to the left until it hits the base at a perpendicular intersection (90 degress).
This distance would be found by subtracting x3 and x1.
The correct answer is the 4th choice in the list.