Direct variation is usually taken to mean a relation of the form "y = kz", where "z" could be any sort of variable expression (such as x or sqrt[x], etc) and "k" is just a number.

Check your textbook for confirmation of the definition. Then rearrange some of the equations, so they're all in the form "y = (something)". Then see if the "something" is of the required form.

Eliz.