Cassandra wants to solve the equation 3z=15 for z in a single step.

To solve the equation \(3z = 15\) for \(z\) in a single step, Cassandra should divide both sides of the equation by \(3\).

So, dividing both sides by \(3\):

\[ \frac{3z}{3} = \frac{15}{3} \]

This simplifies to:

\[ z = 5 \]

Therefore, Cassandra should divide both sides of the equation by \(3\).