| Step | Hyp | Ref | Expression | 
						
							| 1 |  | 2zrng.e |  | 
						
							| 2 |  | 2zrngbas.r |  | 
						
							| 3 | 1 2 | 2zrngamgm |  | 
						
							| 4 |  | elrabi |  | 
						
							| 5 |  | elrabi |  | 
						
							| 6 |  | elrabi |  | 
						
							| 7 | 4 5 6 | 3anim123i |  | 
						
							| 8 |  | zcn |  | 
						
							| 9 |  | zcn |  | 
						
							| 10 |  | zcn |  | 
						
							| 11 | 8 9 10 | 3anim123i |  | 
						
							| 12 |  | addass |  | 
						
							| 13 | 7 11 12 | 3syl |  | 
						
							| 14 | 13 | rgen3 |  | 
						
							| 15 | 1 2 | 2zrngbas |  | 
						
							| 16 | 1 15 | eqtr3i |  | 
						
							| 17 | 1 2 | 2zrngadd |  | 
						
							| 18 | 16 17 | issgrp |  | 
						
							| 19 | 3 14 18 | mpbir2an |  |