When you're solving a question like this, your aim is to get the unknown (in this case 'a') by itself on one side of the equation. You do this by adding/subtracting/multiplying/dividing, whatever is necessary for the question. Note that if you do something to one side of the equation, you must do the same to the other.
6-3a=21
First let's get rid of the 6: subtract 6 from both sides.
6-6-3a=21-6
-3a=15
Now let's get rid of the -3: divide both sides by -3.