If it's not what You are looking for type in the equation solver your own equation and let us solve it.
6-3a=21=
We move all terms to the left:
6-3a-(21)=0
We add all the numbers together, and all the variables
-3a-15=0
We move all terms containing a to the left, all other terms to the right
-3a=15
a=15/-3
a=-5