Answer:
Look in screenshot
Explanation:
So to do this, you have to make sure everything is 100% correct as you go. The thing you have to start with is 12+3=15. You also have to make sure you're crossing out the answers as you use them so you don't get confused.
Hope this helps!
(The stuff highlighted in green is what you did correctly and the numbers in red are the correct answers to things you got wrong in the annotated screenshot below))
[to the people that make sure there is good explanations, please don't delete this answer. This person needs it to stay so they can get a good grade. I explained it the best I could.]