From 4a + 7b - 12c subtract 8a - 9b + 12d.
4a+7b-12c-8a-9b+12dNow you have to combine like terms, so a and a, B and B etc.4a-8a=-4a7b-9b=-2bNo more like term-4a-2b-12c+12dHave a nice day