原式=x^2(3x+5y)^2-y^2(5x+3y)^2
=[x(3x+5y)+y(5x+3y)][x(3x+5y)-y(5x+3y)]
=(3x^2+5xy+5xy+3y^2)(3x^2+5xy-5xy-3y^2)
=(3x^2+10xy+3y^2)(3x^2-3y^2)
=3(x+y)(x-y)(3x+y)(x+3y)
即为所求