First you want to combine like terms, so 3+21 and 8x-6x. You can do this because adding a term to one side of the equation is the same as subtracting it on the other. Then you have 24=2x. Now you can divide the whole thing by 2. Don't forget that if you have multiple terms on one side you have to divide BOTH of them, but in this case you only have one. Now you should have 12=x.
