All you've got to do is some simple algebra! When you solve for a variable, you are trying to get it alone on one side of the equation. So, we want something that looks like y = ...
First, add 21x to both sides of the equation. This will cancel out the -21x on the left side.
-21x + 3y = 6
-21x + 3y + 21x = 6 + 21x
3y = 21x + 6
Now, to get y alone, divide both sides of the equation by 3. This will cancel out the coefficient of 3 on the left side.
3y = 21x + 6
(3y)/3 = (21x + 6)/3
y = 7x + 2
There's the answer. Hope that helps! :)