To go from one compound to another you have to use convert to moles.
Use dimensional analysis to do so.
You start with 100 grams of aluminum, then convert that to moles using the molar mass of aluminum.
Convert moles of aluminum into moles of hydrogen. The balanced chemical equation tells you the mole to mole ratio. Namely, 2 moles Al for every 3 moles H2.
Lastly, now that you have moles of hydrogen, convert that to grams of hydrogen using the molar mass of hydrogen. Hydrogen is diatomic so the molar mass is for 2 hydrogen atoms (2.02g/mol)
