Answer:
12
Explanation:
The constant of proportionality is the slope of the line, the ratio of the y-coordinate to the x-coordinate.
k = 36/3 = 12 . . . . . from the lower left point (not the origin)
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Additional comment
This comes from the equation for a proportion:
y = kx
Solving for k, we get ...
k = y/x
A proportion always graphs as a line through the origin. The slope of it is k, the constant of proportionality.