Okay so this is a science question and need help with! even just how to find the answer!?

A round balloon that has no mass is filled with a mystery gas and is being held in the air by your hand. Your job is to determine the average density of the gas inside the balloon. The balloon has a circumference of 12 cm and the mass of the mystery gas is 32g. Assume that π= 3. Your final units should read g/cm. Does this balloon sink or float when it is released?

Density is mass divided by volume. So as the question tells you the mass is 32g all you need is the volume of the balloon. The balloon is roughly a sphere, so its volume is 32.
Density is 32/32 =1g/cm3
Which is the same as air so balloon would neither sink nor rise (in theory)