So for this you must find the unit rate at which the temperature drops in 1 hour as you are told it is constant. And so if it went down to 60 from 70 in 4 hours then 10/4= -2.5 degrees an hour. Then do 2.5 x 6 to give you that in 6 hours it shall drop 15 degrees which 70-15 gives you 55. Enjoy!
