What should be taken away form 3x2 - 4y2 + 5xy + 20 to obtain -x2 - y2 + 6xy + 20?
Let k should be taken away from 3x2 - 4y2 + 5xy + 20
3x2 - 4y2 + 5xy + 20 - k = -x2 - y2 + 6xy + 20
k = 3x2 - 4y2 + 5xy + 20 - (-x2 - y2 + 6xy + 20)
k = 3x2 - 4y2 + 5xy + 20 + x2 + y2 - 6xy - 20
k = 4x2 - 3y2 - xy