rearrange the 2nd equation
-36a-24b-16c
divide the whole thing by 4 and you'll get
-9a-6b-4c
you'll realize it looks the same as your first equation except that all the + has become minus so the answer will be -7 instead for your new equation
-9a-6b-4c=-7
multiply the whole thing by 4 and you'll get
-36a-24b-16c = -28
so the answer you want is -28