Answer
Concave down
Work Step by Step
The rate of change between $t=1.5$ and $t=2.4$ is
$$
\frac{\Delta R(t)}{\Delta t}=\frac{-3.1-(-5.7)}{2.4-1.5}=2.889
$$
Similarly, we have
$$
\begin{aligned}
& \frac{\Delta H(x)}{\Delta x}=\frac{21.75-21.53}{18-15} \approx 0.073 \\
& \frac{\Delta H(x)}{\Delta x}=\frac{22.02-21.75}{21-18} \approx 0.090 .
\end{aligned}
$$
The rate of change is decreasing, so we expect the graph to be concave down.
