Mohon bantuannya yaa

a.
(5x-9)-(7x+15y)
5x-9-7x+15y
-2x-9+15y
b.
(5x-3y+4)-(5y-3x-4)
5x-3y+4-5y-3x-4
8x-8y+8
c.
(5x^2-9xy-4y^2)-(-x^2-6xy+8y^2)
5x^2-9xy-4y^2+x^2-6xy+8y^2
6x^2-3xy-12y^2