Answer:
[tex]3x^2-9xy+13y^2[/tex]
Step-by-step explanation:
We begin with : [tex]6x^2-7xy+8y^2-[(x^2-3xy-y^2)+(2x^2+5xy-4y^2)][/tex]
First, let us add the two portions within the brackets. To do this, we need to just combine the like terms
[tex]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][/tex]
Next, we need to distribute the negative to all the terms within the brackets
[tex]6x^2-7xy+8y^2-[3x^2+2xy-5y^2]\\\\6x^2-7xy+8y^2-3x^2-2xy+5y^2[/tex]
Now we can combine the like terms again
[tex]6x^2-7xy+8y^2-3x^2-2xy+5y^2\\\\3x^2-9xy+13y^2[/tex]
