Help me please please

Lines a and b are perpendicular. if the slope of line a is 3, what is the slope of line b?

Use the information given below, to compare the cost of operating two different vehicles for one month (4 weeks) you are considering two different cars. you drive to work, a 20 mile round trip, five days a week. gasoline costs you $1.50 per gallon car agets 28 miles per gallon, would have $300 a year in maintenance costs, and would cost you $1,500 per year to insure car b gets 19 miles per gallon, would have $500 a year in maintenance costs, and would cost you $1,000 per year to insure costs car a car b gas cost per month insurance cost per month maintenance cost per month $ total cost per month

Me because i don't understand how to do it.: )