What is the area of the triangle figure below

The area for this triangle is 12mm.
They gave you a the measurement of the sides.
 
First, let look at this triangle where the dots are, is called the adjacent side,    because its the longest side, and the back of the triangle where the 90 degrees is. So to get the length of the adjacent side we look at the length 10 and the length 4. If we subtract the 4 from 10 we get 6. Which is the length of the adjacent side.

Now, if we look at the length twelve, across from it is 8, which should give you the opposite side. So, 12 - 8 = 4. So our opposite side is 8.
 
Finally, we take our 8 side and 12 side and multiply, which is 24.

Now that would be the length of a rectangle, by the way we worked the problem. So, now we divide by two, which gives us 12; half of a rectangle which would look like a triangle.

Answer; 12

The difference is 5,100m?