2log3(x-3y)=log3(3x+y)+log3y
log3(x-3y)^2=log3[y(3x+y)]
(x-3y)^2=y(3x+y)
x^2-6xy+9y^2=3xy+y^2
x^2-9xy+8y^2=0
(x-y)(x-8y)=0
x=y,x=8y
x>3y>0
则x=y显然不成立
所以x=8y
x/y=8