The number multiplying $x$ tells how fast $y$ changes as $x$ increases.

$x$ is in kilometers and $y$ is in watt-hours, so the slope’s units are “watt-hours per kilometer.”

In Desmos, enter $y=45x+120$.

Pick two points on the line, such as $(0,120)$ and $(1,165)$, and compute the rise over run: $\frac{165-120}{1-0}$.

The rise/run value represents how many watt-hours $y$ increases for each additional kilometer $x$.

In the line of best fit $y = 45x + 120$, the coefficient of $x$ (which is 45) is the slope.

Slope means “how much $y$ changes when $x$ increases by 1.”

Here, $x$ is kilometers and $y$ is watt-hours, so a slope of 45 means the energy used increases by about 45 watt-hours for each 1 additional kilometer traveled.

Therefore, the best description is: For each additional kilometer traveled, the robot uses about 45 more watt-hours of energy.