In the equation x=−3y , what is the unit rate?

For the sake of consistency, solve this for y:  -3y = x.

Then, divide both sides by -3:                           y = -x/3

This is equivalent to    y = (-1/3)x.  The "unit rate" here is (-1/3).

The answer would be 27 because assuming that each package has 1 bar of soap and 1 tube of toothpaste, after 27 packages they would run out of toothpaste