

Area of Rectangle  
Area of Triangle A
Technically, each of those distances should be the absolute value of the difference, but the problem is much easier to work without the absolute values. 

Area of Triangle B
Realize however, at if the points don't lie in the same positions (point 2 is the right most and upper most), that the area found using these formulas will be negative. 

Area of Triangle C
For that reason, caution should be exerted to always make the final answer nonnegative. The area of a triangle, after all, can't be negative. 

Let's add the areas of the three outside triangles together.  
Simplifying  
Now, to subtract the areas of the three triangles from the area of the rectangle.  
Simplifying  
Let's regroup those terms  
Wow, quite an exercise, isn't it?  
Now, consider the determinant formed by placing the xcoordinates in the first column, the ycoordinates in the second column, and the constant 1 in the last column.  
Let's evaluate the determinant by expanding along the 3^{rd} column.  
Notice that this differs in the area of the triangle only in sign. The reason for this is because of the order the points were chosen in. If the points were chosen to be points 1, 2, and 3 in a different order, then the determinant would change only in sign. (Recall that interchanging two rows changes the sign of the determinant). 