Application of first order differential equation-tomorrow test

I have a problem of this type of question.

An object thrown into a large body of water cools at a rate proportional to the difference between its temperature and the water temperature. Suppose know that the the water is at a temperature of 27 degrees Celsius. After 4 minutes the object's temperature is 67 degrees, and after 9 minutes the object's temperature is 47 degrees Celsius. What was the temperature of the object when it was thrown into the water?

Differential equation is dQ/dt = -k(Q-Qs)

Q is the temperature of an object

Qs is the water temperature.

Not like other question, i can find K (constant) but in this question K ( constant ) i even cant find it out.