Once you have the slope in terms of $s$, write $y = mx + b$ and plug in one of the points from the table to solve for $b$.

After you find an equation like $y = \text{(something with }s)x + \text{constant}$, clear any fractions by multiplying both sides by $s$ and then rearrange to look like the answer choices (all are in the form $Ax + By = C$).

In Desmos, type s = 2 so that Desmos treats $s$ as the number 2. (You can use any nonzero value, but 2 keeps the numbers simple.)

Enter the three points using $s$: (-2s, 24), (-s, 21), and (s, 15). With $s=2$, these will appear as $(-4,24)$, $(-2,21)$, and $(2,15)$ on the graph.

For each option, type its equation exactly (for example, s x + 3y = 18s for choice A). Desmos will draw four lines. The correct equation is the one whose line passes through all three plotted points; the others will miss at least one point.

Use any two rows from the table to compute the slope $m$.

Between $(-2s,24)$ and $(-s,21)$:

Check with the next pair $(-s,21)$ and $(s,15)$:

So the slope is $m = -\dfrac{3}{s}$.

Use $y = mx + b$ with $m = -\dfrac{3}{s}$.

So the equation is

Plug in one point from the table, for example $(s,15)$:

So in slope-intercept form, the line is

Eliminate the fraction by multiplying both sides by $s$:

Now move $-3x$ to the left side:

This matches choice B, so the correct equation is $3x + sy = 18s$.