Answer
$8.5 ^\circ/sec$
Work Step by Step
Let us consider that $f(t)$ denotes the reading of the thermometer at time $t$.
Then we have $f(0)=-19$ and $f(14)=100$
Let us apply The Mean Value Theorem such that:
$f'(c)=\dfrac{f(b)-f(a)}{b-a}=\dfrac{f(14)-f(0)}{14-0}=\dfrac{119}{14}=8.5 ^\circ/sec$