What is the missing reason in the proof?

Could somebody me with this radical simplification?

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?

Using elimination, what is -4x+2y=-12 4x+8y=-24