Rare Plants Pavlik and Enberg $^{34}$ studied the effect of temperature on a rare species of geyser panic grass. They collected data on temperature effects on the mortality rate of these grasses given in the table.
$$
\begin{array}{|l|cccc|}
\hline \text { Soil Surfacc Tcmpcraturc }\left({ }^{\circ} \mathrm{C}\right) & 18 & 20 & 20 & 23 \\
\hline \text { Mortality rate (\% per week) } & 2.4 & 0.2 & 3.9 & 0.1 \\
\hline \text { Soil Surface Temperature }\left({ }^{\circ} \mathrm{C}\right) & 24 & 27 & 28 & 29 \\
\hline \text { Mortality rate (\% per week) } & 0.8 & 0.2 & 0.1 & 0.7 \\
\hline \text { Soil Surface Temperature }\left({ }^{\circ} \mathrm{C}\right) & 30 & 38 & 39 & 44 \\
\hline \text { Mortality rate (\% per week) } & 0.1 & 0.6 & 1.7 & 6.2 \\
\hline
\end{array}
$$
a. Find the hest-fitting quadratic (as the researches did) relating temperature to the mortality of these grasses.
b. Find where the function attains a maximum.