tolong ya no 6 yang c

kalo ga salah sih, 6x^2-15xy-y^2

=5x^2-9xy-4y^2+x^2-6xy+3y^2
=5x^2+x^2-9xy-6xy-4y^2+3y^3
=6x^2-15xy-y^2

= 5x^2 - 9xy - 4y^2 - ( -x^2 - 6xy + 3y^2 )
= 5x^2 - 9xy - 4y^2 + x^2 + 6xy - 3y^2
= 5x^2 + x^2 - 4y^2 - 3y^2 - 9xy + 6xy
= 6x^2 - 7y^2 - 3xy