Let the angle have measure x°

Then the supplement of the angle = 180° - x°

Since the ratio of the angle to its supplement is 3:7, that means

           angle       3
         ---------- = ---
         supplement    7

              x°       3
         ---------- = ---
          180° - x°    7

Cross-multiply:

                7x° = 3(180° - x°)

                7x° = 540° - 3x°

               10x° = 540°

                 x° = 54°

So its complement is 90° - 54° or 36°

So to find the ratio of the angle 54° to its complement, we
make the fraction:

           angle       54°
         ---------- = ----
         complement    36°

and reduce the fraction 

            54        3 
           ----  to  ---
            36        2

Then change the fraction into the ratio  3:2

Edwin