What you would typically want to do in this type of problem is to solve for the unknown quantity, which in your equation is b.
And whatever you do to one side of the equation, you must also do to the other side. So, if you want to isolate b in the equation 3b = 12, then you would divide both sides by 3. So,
3b/3 = 12/3, which as you see then works out to
b = 4.
* * *
However, if you really do need to add the same number to each side of the equation, then if you chose the number 4 for example, it might look like this:
3b + 4 = 12 + 4
3b + 4 = 16