You are install a light pole in a parking lost. when the pole is positioned properly, it is perpendicular to the pavement. how can you check that the pole is perpendicular using a tape measure

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?