The formula is
Without doing the whole problem for you, is the temperature of what is being cooled at any time , is the temperature of the surrounding area, after you have removed it from being heated.
Are you told ? You did not give it here, and it's pretty important to know.
is the temperature of the object at the moment you have removed it.
Now that you know this, you can use the numbers you are given to solve for the constant which is the cooling constant.
For part a, simply plug in the new value your given using the you just found, and solve for
For part b, set and solve for