SOLUTION: Write an equation of the direct variation that includes the point (9,-12).

Every equation of direct variation is of the form

      y = k·x

where y and x can be any variable and k is a constant number.

Since it must include the point (9,-12) we know that if we
substitute x=9 and y=-12 into y = kx we will have a true
equation, so we do that:

      y = k·x
    -12 = k·9
    -12 = 9k

We divide both sides by 9 to solve for k:

     = 

     = k

So the constant k is , and so we substitute that for k in

      y = k·x

and get the answer:

      y = x

Edwin