Answer:
y = -12x
Explanation:
We assume you're concerned with the form ...
y = kx
Putting -12 for k gives ...
y = -12x . . . . . the first choice
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Additional comment
The answer will depend somewhat on the context of the question. If you're studying proportions, then "k" is the constant of proportionality as shown above.
If you're studying function translations, then "k" is the vertical translation, as in ...
y = m(x -h) +k
In this case, the equation y = x -12 will have a "k" value of -12.