The afferential equation below models the temperature of a $39C$ coup of coffee in a $21C$ reom, where it is known that the coffee cools at a rate of $1C$ per minute men its temperature is $71C.$ Solve the differential equation to find an expression for the temperature of the collee at time t $.($Let $$ be the temperature of the cap of coffee in $*$C and let E be the time in minutek, with $t=0$ corresponding to the time when the temperature was $89C.)$

$\frac{dy}{dt}=-\frac{1}{50}(y-21)$