10x+3y=12 and 10x=-12-3y
~Put equations directly underneath each other.
10x+3y=12
10x=-12-3y
~Move -3y to the other side of the equation so that both x and y are on the same side <- 2nd equation.
10x+3y=-12
10x+3y=12
~subtract equations from one another.
0 = -24
Due to the fact that when you try to add / subtract these equations together and you get zero, there are infinite solutions.
:-)