（1） (4x^3 - 6x^2)÷2x^2= 4x^3÷2x^2 -6x^2 ÷2x^2 = 2x -3
( 2 ) (12m^2 n + 20mn)÷4mn = 12m^2 n ÷4mn + 20mn÷4mn = 3m + 5
( 3 ) (9x^4 - 12x^3 + 54x^2)÷3x^2 = 9x^4÷3x^2 - 12x^3÷3x^3 + 54x^2÷3x^2 = 3x^2 - 4x + 18
( 4 ) (-14x^3 y - 21x^2 y^2 + 63xy^3 ) ÷(-7xy)
= (-14x^3 y)÷(-7xy) -21x^2 y^2÷(-7xy) + 63xy^3÷(-7xy)
= 2x^2 + 3xy -9xy^2