x+y=y+x
x+7=7+x
(x+y)+z=x+(y+z)
2x+4=2*(x+2)
2x=6
x+x=2x
2x+2=12
x+3=2+2x
3x=6
x+4=8
2x+1=1
3+7x=9x-2x+3