What is the value of the expression ? ?

Precalculus question, image attached.

Look at the figure below: triangle abc is a right triangle with angle abc equal to 90 degrees. the length of ac is 5 units and the length of ab is 4 units. d is a point above c. triangle adc is a right triangle with angle dac equal to 90 degrees and dc parallel to ab. what is the length, in units, of segment cd?

Extremely mega ultra important question! plss answer as fast as you can you will get 84 points! the area of this right triangle is half the area of which of these rectangles?