How do you use the coefficient to calculate the number of atoms in each molecule

What type of reaction is shown below? check all that apply. 2h2o2 → 2h2o + o2 synthesis decomposition combustion

Is this correct if not can u correct me

Ablock of aluminum is dropped into a graduated cylinder with an initial volume of water at 75ml and the volumes rises to 90ml. if the block has a mass of 40.5 g what is its density ?