Find how much $y$ changes and how much $x$ changes between those two points, then compute $\frac{\Delta y}{\Delta x}$.

The $x$-axis is in $100s. Think about what an increase of 1 in $x$ means in actual dollars.

In Desmos, define two points on the line of best fit, for example:

$A=(0,18)$

$B=(10,68)$

Compute the slope using an expression:

$m=\frac{y(B)-y(A)}{x(B)-x(A)}$

(Desmos will display a number for $m$.)

Use the value of $m$ as “orders per 1 unit of $x$,” and remember that 1 unit of $x$ equals $100. Choose the statement that matches that rate and those units.

From the graph, the line of best fit passes through approximately $(0,18)$ and $(10,68)$.

So the predicted number of orders increases by about 5 for each 1-unit increase in $x$.

The $x$-axis is measured in $100s, so an increase of 1 in $x$ means an additional $100 spent on ads. Therefore, the slope means that for each increase of $100 in ad spending, the predicted number of orders increases by about 5.

Answer: For each increase of $100 in ad spending, the predicted number of orders increases by about 5.