(3х+у)-(х-4у)
(х+6у)-(8х-у)
(m+n)+(m-n)
(6m+5)-(m-1)
(2+у)-(х+у)
(2m-3n)+(2m+3n)

(y+3x)(4y-x)=4y^2-3x^2(6y+x)(y-8x)=6y^2-8x^2(m+n)(m-n)=m^2-n^2(5+6m)(1-m)=5-6m^2(2+y)(-x-y)=-(2x+y^2)(2m-3n)(2m+3n)=4m^2-9n^2