If f(3) = 15 and f(5) = 45, which of the following could be f(x)?
can anyone explain this one for me?

Just saying, but the rule COULD be:

f(x) = 15(x-2)

720 mm would convert to “72 centimeters”.
You would move the decimal place to the left once.

Also, if you need help with future conversions you can just google the problem for example you would just type “720 mm to cm” and it will give you the answer so you don’t have to post every one. Hope this helped.