（6x+2y)/(9x-3y)=6
(6x/y+2)/(9x/y-3)=6,解得：x/y=5/12
(3x+y)^2/(x^2-y^2)
=(3x/y+1)^2/((x/y)^2-1)
=(81/16)*(24/144)
=243/288