Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings. Let $y(t)$ be the temperature of the water in a pot at the time t minutes. When the water boils the pot is put outside where the temperature is $-20^o$ (sorry about that all Americans).
The temperature $y(t)$ corresponds to the differential equation on the form $y'(t) = k(y(t)+20)$. We also know that the temperature is $40^o$ after 10 minutes.
a) Solve the differential equation. You should use the substitution $u(t) = y(t) + 20 $.
The problem here is that I don't know what they mean by using a substitution.