Complex sentence: Another variation is to have a 26 "front wheel with a 29" rear wheel (commonly called a "69er") though Trek introduced a "69er" in 2007 with a 29 "front wheel and a 26" rear wheel.
Split into simpler sentences: Another variation is to have a 26 "front wheel with a 29" rear wheel (commonly called a "69er"). For example, Trek introduced a "69er" in 2007 with a 29 "front wheel and a 26" rear wheel.