I'm not so sure about number 1. sorry
2. If an investment of 5,000 dollars doubles every 12 years and you have to figure it out for 36 years which 12 can go into 36 3 times, you do 5,000 dollars times 6. you do it times 6 because you have to double it. "an investment of $5,000 "doubles" every 12 "
3. The represent exponential growth and it is 4. (I'm not so good with this type of math but I think this is right. I can't really explain it. Sorry)
4. $520x4% after 5 years would be 36,400 dollars or percent.
Glad to help! :)