唔識數學 -_- Form4的!唔該幫幫手

A mobile telephone company processes 3600 messages every day. At present, each user is charged $1 for a message. The company expects in the future that for svery $0.1 increase in the service charge of a message, the decrease in number of calls os 120 per day.
If the service charge for a message is increase by $x in the future,find,in trems of x
　i) the new service charge for a message,
　ii) the expected number of messages per day,