Sorry the answer comes so late,

so we have a function : changing the price has an influence on the number of tents sold so the price P of one tent is the x variable and the number sold N is the y variable. N is a function of P.

This is linear :

We write the points (x,y) as being (P,N)

we have (550$,150) and then (600$,145),(650$,140)... If you draw the points you'll see it is linear. The slope can be taken for any two points since the function is linear and the slope is always the same.

We take the two points (550$,150) and (600$,145)

slope = (145-150)/(600-550)=-5/50=-0.1

So N = -0.1 P + b where b is not known (intersection with the y axes). We replace by one of the points :

150 = -0.1 * 550 + b so we isolate b and get b=205. So N=-0.1 P + 205.

Some info about how they calculate their revenue is missing to continue the answer.