Hi, I think what you need to do is multiply 21 x 4 x 1/3. If you need help multiplying there’s a helpful website called Calculator Soup that is very helpful.
For the equation to work, you have to have the same number on both sides of the equal sign
so to find ?, you can do:
13 - 2 = 12 - ? Simplify
11 = 12 - ? Subtract 12 on both sides
-1 = - ? Multiply -1 on both sides
1 = ?

Look at the picture.
The common part does not exist.