Answer:

Explanation:
We begin with :
![6x^2-7xy+8y^2-[(x^2-3xy-y^2)+(2x^2+5xy-4y^2)]](https://img.qammunity.org/2021/formulas/mathematics/middle-school/90f4gfc3yq0q43bet8zink7d0hxn88thld.png)
First, let us add the two portions within the brackets. To do this, we need to just combine the like terms
![6x^2-7xy+8y^2-[(x^2-3xy-y^2)+(2x^2+5xy-4y^2)]\\\\6x^2-7xy+8y^2-[3x^2+2xy-5y^2]](https://img.qammunity.org/2021/formulas/mathematics/middle-school/qyqckbdan7uvtjc9uau6csu21epyt03stl.png)
Next, we need to distribute the negative to all the terms within the brackets
![6x^2-7xy+8y^2-[3x^2+2xy-5y^2]\\\\6x^2-7xy+8y^2-3x^2-2xy+5y^2](https://img.qammunity.org/2021/formulas/mathematics/middle-school/76aahxdo2czl574e51la085xqujwwefszj.png)
Now we can combine the like terms again
