How would i solve questions like this?

Calculate the molar mass of aluminum oxide (al2o3). express your answer to four significant figures.

Find the mass in grams of 1.37x1020 particles of h3po4

Sally is making a model of a magnesium atom with an atomic mass number of 24 for her chemistry class. she has foam balls for the protons, neutrons, and electrons. she has added 6 neutrons to her model so far. how many more neutrons does she need to add to complete her neutral atom of magnesium?