3x = 12 - 6y....reduces to x = 4 - 2y...rearranged is x + 2y = 4
so now we have :
x + 2y = 3
x + 2y = 4
ok....I dont have to go any farther to know that this has no solution because ur equations have the same slope and different y int, this means ur lines are parallel and have no solution because they never cross each others path.
