(1/2x-1/3y)(1/2x+1/3y)(1/4x^2+1/9y^2)
=[(1/2x)^2-(1/3y)^2](1/4x^2+1/9y^2)
=(1/4x^2-1/9y^2)](1/4x^2+1/9y^2)
=(1/4x^2)^2-(1/9y^2)^2
=1/16x^4-1/81y^2