If 3x+6x+3=21, find the value of x

To start, i would put all the x's on one side of the equation and the two numbers on the other side of the equation. However be careful when moving the 3 as when we move it over the equals symbol its sign inverts and it becomes a minus 3. For example if we had x=1, then x-1=0 as the 1 has swapped sides of the equation. So for our equation this would give 3x+6x=21-3. Then i would tidy this up a bit by adding and subtracting giving the equation 9x=18. This is then the same as saying 9 (times by something) = 18. However we only want to find out what one of those "times by somethings" is. So in this case we would divide by 9. Similar to before we would have to divide both sides by 9 leaving us with x=2.