9x mean 9 multipled with an unkown number and 9x is subtracted from another unknown number 'y' . you can substitute with any letters you want ,but you have to use two different letters.
Answer:
I got - 5 as an answer.
I don't think I made a mistake.
Please if you find any error, contact me in the comment section.
Step-by-step explanation:

I can help you with #20. For example, lets say 3/5, this fraction would be made up of 5 parts and 3 are covered, which doesn't make it full, if it was 5/5 it would be full. When you do something with a reciprocal you turn it over, so 3/5 would be turned over to 5/3, thus making it a whole number
4000 ........................................