tolong nomor 28,29,30 thank you

28.
[tex]4 {x}^{2} + 4xy - 5 {y}^{2} - 9 {x}^{2} + 3xy + 6 {y}^{2} = \\ - 5 {x}^{2} + 7xy + {y}^{2} [/tex]
29. 2a+3b-5+6a-4b-5+9=
2a+6a+3b-4b-5+9=
8a-b+4

30.
[tex] {b}^{2} + ab - 3 {a}^{2} - {a}^{2} - ab - {b}^{2} = \\ {b}^{2} - {b}^{2} + ab - ab - 3 {a}^{2} - {a}^{2} = \\ - 4 {a}^{2} [/tex]