離散習題3-123 (c)

題目：Let A={a, b, c, d} , B={1, 2, 3, 4, 5, 6, 7}
how many onto functions f: B->A satisfying f(1)=a?

解答是onto(6,3)+onto(6,4)=2100
為什麼這題不能用全部onto個數 - f(1)!=a
=> onto(7,4) - 3*onto(6,4)=3720