The quadrilateral shown below has 3 of its sides of length 3, 4, and 12.
The angles marked are right angles.       Find the fourth side.



Once found, show that the sum of the areas of the squares of the three shorter
sides is equal to the square of the longest side.
That is, show that the sum of the areas of the green , blue and yellow
regions in the figure below equals the area of the red region.