There is no property that justifies it. The equation is true for some values of x, not for others.
3x plus 9 equals what?
3x + 2 = 3x + 6 This is not possible.
Their graphs are.
what is a slope
4x plus 7 plus 3x equals 19 plus xx = 2
2x+3x = 5x
-46 = 3x + 7x
50, 53, 59