Hanna hypothesizes that granite is denser than sandstone. How can Hanna test her hypothesis?

To test if the hypothesis is correct, a good way is to think of it this way:

Density = mass/volume, right?

Calculate the mass and volume of each and do the equation; this will test your hypothesis.

You will be left with the density of each. But, make sure that the sample sizes are the same (controlled variable) otherwise it will be an unfair test.