First you can either substitute or elimination.
So what I did was (2) -6x+2y=16 -------> -12x+4y=32
(-3) -4x+y=9 -------------------------------------> 12x-3y= -27
which would come to y=5 and if you put it back in both equations which would make x= -1
4 cups of milk to 1 cup of oatmeal