1/7(x-y)^3+7(y-x)=1/7[(x-y)^3-49(x-y)]=1/7(x-y)[(x-y)^2-49]=1/7(x-y)(x-y+7)(x-y-7)16(a+2b)^2-9(a-2b)^2 =[4(a+2b)-3(a-2b)][4(a+2b)+3(a-2b)]=(a+14b)(7a-2b)