Newton's Law of cooling?? HELP!!

This problem is the law just backwards. There are also two unknown constants and i therefor have no idea how to manipulate the equation when two are missing...

Question:

On a hot day a thermometer is taken outside from an air-conditioned room where the temperature is 21 degrees C. After one minutes it reads 27 degrees C, and after two minutes it reads 30 degrees C. Use Newton's law of cooling to find the outdoor temperature.

My work so far:

dy/dx = K(y-?) ?=outdoor temperature y=thermometer reading

21=(21-?) e^K(0)

27=(27-?) e^K(1)

30=(30-?) e^K(3)

What should I do??? I really need guidance on this problem!!!!!!!!