Transformation of a linear graph

The figure shows the graphs of the function y = f(x) and y = g(x).  The four indicated points all have integer coordinates.

If g(x) = k ∙ f(x), what is the value of k?

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On f(x), the two indicated points are (0,1) and (1,-1)
On g(x), the two indicated points are (0,-3) and (1,3)

Think of what the y-coordinates on f(x) are multiplied by,
in order to get the y-coordinates on g(x).

On f(x), 1 is multiplied by -3 to get -3 on g(x).
and
On f(x) -1 is multiplied by -3 to get 3 on g(x).

Therefore if g(x) = k ∙ f(x), then the value of k is -3.

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