First, your formula is wrong. At least one of the "t" should be "T", the temperature of the environment, 70 degrees, while at least one should be "t" the time after the object is removed from cooling.

If I am reading this correctly, your formula should be where "T" is 70 and "t" is the number of minutes after the object is removed from cooling.

At t= 0, when the object is first taken out of cooling, U(0)= 28 so [tex]28= 70+ (U0- 70)e^{k(0)}= 70+ (U0- 70)= U0. That tells you what U0 is.

At t= 10, 10 minutes after being taken out of cooling . You already know U0 so you can solve that for k (you will need to use a logarithm).

Finally, after you know U0 and k, evaluate