Answer:
Image result for Three vertices of a rectangle are (6,1), (3,1), and (3,8). What are the coordinates of the fourth vertex?
One would be to divide the rectangle into two triangles, then use the Pythagorean Theorem to find the distances between the points, and finally use Heron's formula to find the areas of the two triangles.
To find the area of a triangle where you know the x and y coordinates of the three vertices, you'll need to use the coordinate geometry formula: area = the absolute value of Ax(By - Cy) + Bx(Cy - Ay) + Cx(Ay - By) divided by 2.
Explanation: