Answer
$8.5 ^\circ/sec$
Work Step by Step
Suppose $f(t)$ is the reading of the thermometer at time $t$.
Thus, $f(0)=-19$ and $f(14)=100$
Need to use The Mean Value Theorem which states that
$f'(c)=\dfrac{f(b)-f(a)}{b-a}$
and $f'(c)=\dfrac{f(14)-f(0)}{(14-0)}=\dfrac{119}{14}=8.5 ^\circ/sec$