What should be added to `2xy-3x^(2)+6` so that the sum is `4x^(2)-7xy+15`?

Answer

`7x^(2) - 9xy +9`

Explanation

Let f(x) should be added to `2xy-3x^(2)+6`, so tha rthe sum is `4x^(2)-7xy+15`
`f(x) =4x^(2)-7xy+15-(2xy-3x^(2)+6)`
`=4x^(2)-7xy+15-2xy+3x^(2)-6`
`(4x^(2)+3x^(2))+(-7xt-2xy)+(15-6)`
`=7x^(2)-9xy+9`