-3 + 2x = -x + 6 Add x to both sides
-3 + 3x = 6 Add 3 to both sides
3x = 9 Divide both sides by 3
x = 3
-3 + 2x = -x + 6 Plug in 3 for all of the x's
-3 + 2(3) = -3 + 6 Multiply 2 and 3
-3 + 6 = -3 + 6
Since both sides of the equation are the same you know that 3 made the equation equivalent.