Answer
$Y(t)=300- 300\cdot (0.97)^t$
Work Step by Step
The problem describes an exponential function with the following parameters.
\begin{equation}
\begin{aligned}
a&= 300\\
r&= 3\%= 0.03\\
b&=1-r = 0.97
\end{aligned}
\end{equation} Note that the yam's temperature is the difference between the oven's initial temperature and the temperature difference between the oven and the yam's temperature.
Hence, a model of the yam's temperature is
\begin{equation}
\begin{aligned}
Y(t)&=a-a\cdot b^t\\
&=300- 300\cdot (0.97)^t\\
\end{aligned}
\end{equation}
