With Newton's Law of Cooling, you can either remember the formula or remember the idea behind it and derive it. It is kind of a pain to derive under pressure if you haven't done it a few times, so maybe you should just memorize it when you have to use it.
The end result is:
You are given T_0 and T_a, the starting temp for the body and the ambient, or outside temperature. The only thing left to find is k, the decay rate. Since you are given information about the temperature after a certain time, t, you can plug everything in except for the missing k. Solve for k.