Sadly, that is the point of the problem.

This one is nearly written for you.

If T(t) is the temperature, what is the CHANGE in temperature? It is dT/dt or T'(t) - the first derivative.The rate at which the coffee cools down

Our lives are simplified!Write T instead of T(t).

So we have dT/dt = ????

Remember what "proportional" means? This leaves us with:is proportional to

dT/dt = k*(????)

What? Oh, they told us to use "C". Fine.

dT/dt = C*(????)Use C as your proportionality constant.

We're given the ambient temperature, so now we have:the difference between the temperature of the coffee and the room temperature.

dT/dt = k(|T - 17ºC|)

Now what?