A direct variation equation has the general form:
y = kx
where k is the constant of proportionality. If you have one point, (x,y), you can find k:
y/x = k
We have one point, (-3,9), so:
9/(-3) = k = -3
Now you can find the value of x for y = 6:
y/x = k
y/k = x
6/(-3) = x
Solve for x.
