Answer :

First, we find the slope of the function

[tex]m=\frac{y_2-y_1}{x_2-x_1}[/tex]

Let's replace the points (0,25) and (21,10).

[tex]m=\frac{10-25}{21-0}=\frac{-15}{21}=-\frac{5}{7}[/tex]

This means the function is decreasing 5 units vertically, while it's increasing 7 units horizontally.

Hence, the function is linear.

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