Question 12840
You were on the right track till you you got to the step of subtracting the 9 from both sides. 
You have 21x-9=12x+18
Which actually is 21x+(-9)=12x+18
Remember to get rid of the 9 and make it a zero you need to add a POSITIVE 9 to both sides. 
So you get: 21x-9+9=12x+18+9
This cancels out the 9 from the left side;
21x=12x+18+9;
21x-12x=27
9x=27
x=3
Hope this helps you with your algebra, it really isn't that difficult.
=)