Hey there!
For this problem, you can go ahead and split up your equation to replicate the format that you've been provided in your first two equations.
–4a + 6x – 4c + 6y – 4b
a + b + c = (–9)
–4a – 4b – 4c = –4(–9)
–4a – 4b – 4c = 36
x + y = 8
6x + 6y = 6(8)
6x + 6y = 48
If you don't understand what I just did, consider what I have below (this is not part of your problem, this is just so you can understand how I got the answers for the individual equations above):
x = 6
2x = 2(6) = 12
3x = 3(6) = 18, etc...
So, your big equation should be equal to 48 + 36, which is 84.
–4a + 6x – 4c + 6y – 4b = 84
Hope this helped you out! :-)