At a discount store, items are discounted by the same amount each week. The graph shows the relationship between time and cost of an item with an original cost of $300. At what rate is the item discounted each week?

Ah hah !

Did the magic word "rate" jump out at you ? When you see the
word 'rate' ... especially in connection with a graph ... that's just
a code word for "slope".

So what is the slope of this graph ?

-- Pick any two points on the line. It doesn't matter where
they are on the line, or how far apart.

-- Find the distance that the line rises vertically, as it goes
left to right between the two points. (If it drops from left to
right, then this amount is negative.)

-- Find the distance that the line goes horizontally, as it goes
left to right between the two points.

-- The slope of the line is ...

(vertical distance between the points)
divided by
(horizontal distance between the points).

On this graph, let's use the ends of the blue line for our two points.

-- Vertical rise between the two points = ($225 - $300) = -$75 .

-- Horizontal distance between the two points = (5 weeks - 0 weeks)

   = 5 weeks .

Slope of the line = (-75 dollars) / (5 weeks)

   = (-75/5)   (dollars/weeks)

   = -15 dollars/week .

There's the amount the item price changes each week.