Your picture has several issues.
- x is not defined. It appears to be the number of apartments rented.
- R(x) is defined two different ways. The first way, it looks like it is the revenue from a single apartment. The second way, it looks like it is the revenue from the entire apartment complex.
- The derivative is in error. It should be -20x +2000. In any event, this is not the derivative you want. You're not trying to maximize revenue; you're trying to maximize profit.
- It might be useful to write an equation for profit: P(x) = R(x) -200x = -10x² +1800x. Then when you go to maximize it, your derivative will be P'(x) = 0 = -20x +1800 ⇒ x = 90.
Your answer is correct, but the path you followed to get there has a few potholes.
Answer:
1st step, DON'T PANIC. You have plenty of time to study. When it's a big exam, I like to set aside 2 hours daily for studying. If you're confused by certain topics, watch a video or ask your professor how to properly complete the answer. One college tip I learned: ALWAYS ASK THE PROFESSOR IF YOU GET CONFUSED WHILE STUDYING! They can help clear up problems you have. On the night before the exam, make sure you get a good night's rest. On the day of your exam, make sure to eat a good breakfast.
You have plenty of time, you got this! :)
