3ab+4c=72ab, jika a=6-4c maka c

a=6-4c

3ab+4c=72ab

3b(6-4c) + 4c = 72b(6-4c)

18b – 12bc + 4c = 432b – 288bc

432b – 18b -288bc + 12bc – 4c = 0

414b – 276bc – 4c = 0

276bc + 4c = 414b

c(276b + 4) = 414b

c = 414b / (276b + 4)