12a + 6b + 9a + 14b =(12a + 9a) + (6b + 14b) =21a + 20b
If it's a Conn trumpet, it's the model number. The Conn 20B is a student model horn.
The Electric Company - 1971 20B 6-20 was released on: USA: 12 November 1976
20B to 20 KB
-(5b - 4)(25b2 + 20b + 16)