To find out how much pizza is left, we need to add the amounts that Lily and Rose ate and then subtract that from the whole pizza.
Lily ate three ninths (3/9) and Rose ate four ninths (4/9).
Now, we add their portions: \[ \frac{3}{9} + \frac{4}{9} = \frac{3 + 4}{9} = \frac{7}{9} \]
They together ate seven ninths of the pizza.
To find out how much is left, we subtract the amount eaten from the whole pizza (1, or 9/9): \[ 1 - \frac{7}{9} = \frac{9}{9} - \frac{7}{9} = \frac{2}{9} \]
So, the amount of pizza left is \( \frac{2}{9} \).
Therefore, the answer is \( \frac{2}{9} \).