Hey there!
Here is your question -

In this equation, you're looking for n alone. To keep the variable alone, you have to do the opposite of what's happening to the variable.
In this equation, 12 is being added to n. The opposite of that is subtracting 12.
Subtract 12 from both sides -

Your answer is -8.
Hope this helps! :)
If we use the width as x we have 2x with both widths and 4x with both lengths. So 6x=24 and x=4. So length is 8 and width is 4.