Now multiply x-y and x^2+y^2 together to get:
(x-y)(x^2+y^2)=x^3-y^3+xy^2-yx^2=(x^3-y^3) + xy(y-x)
But the the term at the extreme left is equal to 2.8=16, and the term at the
extreme right is (x^3-y^3)-4, hence x^3-y^3=20.
Please check the algebra carefully to make sure it's correct.