ok first you need to put like with like so let's separate this, x with x and y with y.
6x+9x -8y+4y.
ok let's start with the x
6x+9x=15x
now the y. WARNING the 8y is negative so be careful there!
-8y+4y=-4y see? your answer is negative four
now put them together 15x-4y
OR
-4y+15x
hope this helps:)
good luck!