To solve any algebraic equation, we need to get x by itself. The first step to doing that is doing the multiplication (since we can't simplify the parenthesis any further, and because there are no exponents).
Simplifying the parenthesis gives us:
12x - 66 + 15 = 21
Now we want to get all terms with x on one side and all the numbers on the other.
12x = 21 + 66 - 15
12x = 72
Now we want to get x by itself (meaning that we want to get a <em>single</em> x). We can do that by dividing both sides by the number in front of x.