Differential equations: Newton's Law of Coolin

Need help on this..DE Newton's Law of Cooling

A thermometer is removed from a room whose ambient temperature is 70° F and is taken outside.
After one minute the thermometer reads 60° F and after three minutes, it reads 55° F. What is the outside
temperature (which is assumed to be constant)? Find your answer by solving DE model of Newton’s Law
of cooling.